Rho GTPases in platelet function

Inhibition of RHOA activity preserves the survival and hemostasis function of long-term cold-stored platelets

ABSTRACT

Patients with thrombocytopenia require platelet transfusion to prevent and stop hemorrhage. Cold storage of platelets results in complex molecular lesions, including changes in membrane microdomains that are recognized by host macrophages and hepatocyte counter-receptors, resulting in phagocytosis and clearance upon transfusion. For this reason, platelets are stored at room temperature, a method that confers increased risk of bacterial contamination. By applying signaling analysis and genetic and pharmacological approaches, we identified that cold-induced activation of RAS homolog family, member A (RHOA) GTPase causes the major hallmarks of platelet cold storage lesions. RHOA deficiency renders murine platelets insensitive to cold storage-induced damage, and pharmacological inhibition by a RHOA activation inhibitor, R-G04, can prevent the cold storage-induced lesions. RHOA inhibition prevents myosin activation and clathrin-independent formation and internalization of lipid rafts enriched in active glycosyltransferases as well as abnormal distribution of GPIbα. RHOA inhibition further prevents the metabolic reprogramming of cold storage-induced lesions and allows the maintenance of glycolytic flux and mitochondria-dependent respiration. Importantly, human platelets transfused in mice after cold storage, in the presence of R-G04 or its more potent enantiomer S-G04, can circulate in vivo at similar levels as room temperature-stored platelets while retaining their hemostatic activity in vivo, as assessed by bleeding time correction in aspirin-treated mice. Our studies provide a mechanism-based translational approach to prevent cold storage-induced damage, which is useful for human platelet transfusion in patients with thrombocytopenia.

How Protein Depletion Balances Thrombosis and Bleeding Risk in the Context of Platelet's Activatory and Negative Signaling

Platelets are small cell fragments that play a crucial role in hemostasis, requiring fast response times and fine signaling pathway regulation. For this regulation, platelets require a balance between two pathway types: the activatory and negative signaling pathways. Activatory signaling mediators are positive responses that enhance stimuli initiated by a receptor in the platelet membrane. Negative signaling regulates and controls the responses downstream of the same receptors to roll back or even avoid spontaneous thrombotic events. Several blood-related pathologies can be observed when these processes are unregulated, such as massive bleeding in activatory signaling inhibition or thrombotic events for negative signaling inhibition. The study of each protein and metabolite in isolation does not help to understand the role of the protein or how it can be contrasted; however, understanding the balance between active and negative signaling could help develop effective therapies to prevent thrombotic events and bleeding disorders.

A Platelet Reactivity ExpreSsion Score derived from patients with peripheral artery disease predicts cardiovascular risk

Platelets are key mediators of atherothrombosis, yet, limited tools exist to identify individuals with a hyperreactive platelet phenotype. In this study, we investigate the association of platelet hyperreactivity and cardiovascular events, and introduce a tool, the Platelet Reactivity ExpreSsion Score (PRESS), which integrates platelet aggregation responses and RNA sequencing. Among patients with peripheral artery disease (PAD), those with a hyperreactive platelet response (>60% aggregation) to 0.4 µM epinephrine had a higher incidence of the 30 day primary cardiovascular endpoint (37.2% vs. 15.3% in those without hyperreactivity, adjusted HR 2.76, 95% CI 1.5-5.1, p = 0.002). PRESS performs well in identifying a hyperreactive phenotype in patients with PAD (AUC [cross-validation] 0.81, 95% CI 0.68 -0.94, n = 84) and in an independent cohort of healthy participants (AUC [validation] 0.77, 95% CI 0.75 -0.79, n = 35). Following multivariable adjustment, PAD individuals with a PRESS score above the median are at higher risk for a future cardiovascular event (adjusted HR 1.90, CI 1.07-3.36; p = 0.027, n = 129, NCT02106429). This study derives and validates the ability of PRESS to discriminate platelet hyperreactivity and identify those at increased cardiovascular risk. Future studies in a larger independent cohort are warranted for further validation. The development of a platelet reactivity expression score opens the possibility for a personalized approach to antithrombotic therapy for cardiovascular risk reduction.

[Platelet-specific Rictor knockout inhibits platelet production and activation and reduces thrombosis in mice]

OBJECTIVE

To investigate the effects of platelet-specific Rictor knockout on platelet activation and thrombus formation in mice.

METHODS

PF4-Cre and Rictorfl/fl transgenic mice were crossed to obtain platelet-specific Rictor knockout (Rictor-KO) mice and wild-type mice (n=65), whose expression levels of Rictor, protein kinase B (AKT) and p-AKT were detected using Western blotting. Platelet counts of the mice were determined using routine blood tests, and hemostatic function was assessed by tail vein hemorrhage test. Venous thrombosis models were established in the mice to evaluate the effect of Rictor knockout on thrombosis. Platelet aggregation induced by ADP and thrombin was observed in Rictor-KO and wild-type mice, and flow cytometry was used to analyze the expression levels of integrin αIIbβ3 and CD62P in resting and activated platelets. Plasma PF4 levels were determined with ELISA. Megakaryocytes from Rictor-KO and wild-type mice were incubated by vWF immunohistochemical antibody and APC-CD41 antibody to detect the number and ploidy of megakaryocytes, respectively. Platelet elongation on collagen surface was observed with scanning electron microscopy.

RESULTS

Compared with the wild-type mice, Rictor-KO mice showed significantly decreased AKT phosphorylation, decreased platelet production, reduced thrombosis, and decreased platelet activation in response to ADP and thrombin stimulation. The Rictor-KO mice also showed lowered expression level of P-selectin protein and activation of integrin αIIbβ3 with suppression of platelet extension, reduced plasma PF4 level and decreased number of megakaryocytes in the bone marrow. The ploidy of megakaryocytes and the mean area of proplatelets were both significantly decreased in Rictor-KO mice.

CONCLUSION

Platelet-specific Rictor knockout inhibits platelet generation and activation to result in decreased thrombus formation in mice, suggesting the potential of mTORC2 activity inhibition as an efficient antithrombotic strategy.

Increase of Phosphoprotein Expressions in Amotosalen/UVA-Treated Platelet Concentrates

Background

Pathogen inactivation treatment (PIT) has been shown to alter platelet function, phenotype, morphology and to induce a faster aging of platelet concentrates (PCs). Key pieces of information are still missing to understand the impacts of PITs at the cellular level.

Objectives

This study investigated the impact of amotosalen/UVA on PCs, from a post-translational modifications (PTM) point of view. Phosphoproteomic analyses were conducted on resting platelets, right after the amotosalen/UVA treatment and compared with untreated PCs.

Method

A two-arm study setting was carried out to compare PIT (amotosalen/UVA) to untreated PCs, on day 1 post-donation. Based on a pool-and-split approach, 12 PCs were split into two groups (treated and untreated). Quantitative phosphoproteomics was performed using TMT technology to study the changes of phosphoproteins right after the PIT.

Results

A total of 3,906 proteins and 7,334 phosphosites were identified, and 2,473 proteins and 2,214 phosphosites were observed in at least 5 to 6 replicates. Compared to untreated platelets, PIT platelets exhibited an upregulation of the phosphorylation effects, with 109 phosphosites identified with a higher than 2-fold change. Two pathways were clearly identified. The mitogen activated protein kinases (MAPKs) cascade, which triggers the granule secretion and the activation of the pS15 HSPB1. One of the shape change pathways was also observed with the inhibition of the Threonine 18 and Serine 19 phosphorylations on myosin light chain (MLC) protein after the amotosalen/UVA treatment.

Conclusions

This work provides a deep insight into the impact of amotosalen/UVA treatment from a phosphoprotein viewpoint on resting platelets. Clear changes in phosphorylation of proteins belonging to different platelet pathways were quantified. This discovery corroborates previous findings and fills missing parts of the effect of photochemical treatments on platelets.

Ultra-high throughput-based screening for the discovery of antiplatelet drugs affecting receptor dependent calcium signaling dynamics

Distinct platelet activation patterns are elicited by the tyrosine kinase-linked collagen receptor glycoprotein VI (GPVI) and the G-protein coupled protease-activated receptors (PAR1/4) for thrombin. This is reflected in the different platelet Ca2+ responses induced by the GPVI agonist collagen-related peptide (CRP) and the PAR1/4 agonist thrombin. Using a 96 well-plate assay with human Calcium-6-loaded platelets and a panel of 22 pharmacological inhibitors, we assessed the cytosolic Ca2+ signaling domains of these receptors and developed an automated Ca2+ curve algorithm. The algorithm was used to evaluate an ultra-high throughput (UHT) based screening of 16,635 chemically diverse small molecules with orally active physicochemical properties for effects on platelets stimulated with CRP or thrombin. Stringent agonist-specific selection criteria resulted in the identification of 151 drug-like molecules, of which three hit compounds were further characterized. The dibenzyl formamide derivative ANO61 selectively modulated thrombin-induced Ca2+ responses, whereas the aromatic sulfonyl imidazole AF299 and the phenothiazine ethopropazine affected CRP-induced responses. Platelet functional assays confirmed selectivity of these hits. Ethopropazine retained its inhibitory potential in the presence of plasma, and suppressed collagen-dependent thrombus buildup at arterial shear rate. In conclusion, targeting of platelet Ca2+ signaling dynamics in a screening campaign has the potential of identifying novel platelet-inhibiting molecules.

Exploring bias in platelet P2Y1 signalling: Host defence versus haemostasis

Platelets are necessary for maintaining haemostasis. Separately, platelets are important for the propagation of inflammation during the host immune response against infection. The activation of platelets also causes inappropriate inflammation in various disease pathologies, often in the absence of changes to haemostasis. The separate functions of platelets during inflammation compared with haemostasis are therefore varied and this will be reflected in distinct pathways of activation. The activation of platelets by the nucleotide adenosine diphosphate (ADP) acting on P2Y1 and P2Y12 receptors is important for the development of platelet thrombi during haemostasis. However, P2Y1 stimulation of platelets is also important during the inflammatory response and paradoxically in scenarios where no changes to haemostasis and platelet aggregation occur. In these events, Rho-GTPase signalling, rather than the canonical phospholipase Cβ (PLCβ) signalling pathway, is necessary. We describe our current understanding of these differences, reflecting on recent advances in knowledge of P2Y1 structure, and the possibility of biased agonism occurring from activation via other endogenous nucleotides compared with ADP. Knowledge arising from these different pathways of P2Y1 stimulation of platelets during inflammation compared with haemostasis may help therapeutic control of platelet function during inflammation or infection, while preserving essential haemostasis. LINKED ARTICLES: This article is part of a themed issue on Platelet purinergic receptor and non-thrombotic disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.4/issuetoc.

Atorvastatin-mediated inhibition of prenylation of Rab27b and Rap1a in platelets attenuates their prothrombotic capacity and modulates clot structure

Statins inhibit the mevalonate pathway by impairing protein prenylation via depletion of lipid geranylgeranyl diphosphate (GGPP). Rab27b and Rap1a are small GTPase proteins involved in dense granule secretion, platelet activation, and regulation. We analyzed the impact of statins on prenylation of Rab27b and Rap1a in platelets and the downstream effects on fibrin clot properties. Whole blood thromboelastography revealed that atorvastatin (ATV) delayed clot formation time (P < .005) and attenuated clot firmness (P < .005). ATV pre-treatment inhibited platelet aggregation and clot retraction. Binding of fibrinogen and P-selectin exposure on stimulated platelets was significantly lower following pre-treatment with ATV (P < .05). Confocal microscopy revealed that ATV significantly altered the structure of platelet-rich plasma clots, consistent with the reduced fibrinogen binding. ATV enhanced lysis of Chandler model thrombi 1.4-fold versus control (P < .05). Western blotting revealed that ATV induced a dose-dependent accumulation of unprenylated Rab27b and Rap1a in the platelet membrane. ATV dose-dependently inhibited ADP release from activated platelets. Exogenous GGPP rescued the prenylation of Rab27b and Rap1a, and partially restored the ADP release defect, suggesting these changes arise from reduced prenylation of Rab27b. These data demonstrate that statins attenuate platelet aggregation, degranulation, and binding of fibrinogen thereby having a significant impact on clot contraction and structure.

The role of heat shock proteins in the pathogenesis of heart failure (Review)

The influence of heat shock proteins (HSPs) on protein quality control systems in cardiomyocytes is currently under investigation. The effect of HSPs on the regulated cell death of cardiomyocytes (CMCs) is of great importance, since they play a major role in the implementation of compensatory and adaptive mechanisms in the event of cardiac damage. HSPs mediate a number of mechanisms that activate the apoptotic cascade, playing both pro‑ and anti‑apoptotic roles depending on their location in the cell. Another type of cell death, autophagy, can in some cases lead to cell death, while in other situations it acts as a cell survival mechanism. The present review considered the characteristics of the expression of HSPs of different molecular weights in CMCs in myocardial damage caused by heart failure, as well as their role in the realization of certain types of regulated cell death.

Beyond epithelial damage: vascular and endothelial contributions to idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disease of the lung with poor survival. The incidence and mortality of IPF are rising, but treatment remains limited. Currently, two drugs can slow the scarring process but often at the expense of intolerable side effects, and without substantially changing overall survival. A better understanding of mechanisms underlying IPF is likely to lead to improved therapies. The current paradigm proposes that repetitive alveolar epithelial injury from noxious stimuli in a genetically primed individual is followed by abnormal wound healing, including aberrant activity of extracellular matrix-secreting cells, with resultant tissue fibrosis and parenchymal damage. However, this may underplay the importance of the vascular contribution to fibrogenesis. The lungs receive 100% of the cardiac output, and vascular abnormalities in IPF include (a) heterogeneous vessel formation throughout fibrotic lung, including the development of abnormal dilated vessels and anastomoses; (b) abnormal spatially distributed populations of endothelial cells (ECs); (c) dysregulation of endothelial protective pathways such as prostacyclin signaling; and (d) an increased frequency of common vascular and metabolic comorbidities. Here, we propose that vascular and EC abnormalities are both causal and consequential in the pathobiology of IPF and that fuller evaluation of dysregulated pathways may lead to effective therapies and a cure for this devastating disease.

Protease-activated receptors and glycoprotein VI cooperatively drive the platelet component in thromboelastography

BACKGROUND

Thromboelastography (TEG) is used for real-time determination of hemostatic status in patients with acute risk of bleeding. Thrombin is thought to drive clotting in TEG through generation of polymerized fibrin and activation of platelets through protease-activated receptors (PARs). However, the specific role of platelet agonist receptors and signaling in TEG has not been reported.

OBJECTIVES

Here, we investigated the specific receptors and signaling pathways required for platelet function in TEG using genetic and pharmacologic inhibition of platelet proteins in mouse and human blood samples.

METHODS

Clotting parameters (R time, α-angle [α], and maximum amplitude [MA]), were determined in recalcified, kaolin-triggered citrated blood samples using a TEG 5000 analyzer.

RESULTS

We confirmed the requirement of platelets, platelet contraction, and αIIbβ3 integrin function for normal α and MA. Loss of the integrin adaptor Talin1 in megakaryocytes/platelets (Talin1mKO) also reduced α and MA, but only minimal defects were observed in samples from mice lacking Rap1 GTPase signaling. PAR4mKO samples showed impaired α but normal MA. However, impaired TEG traces similar to those in platelet-depleted samples were observed with samples from PAR4mKO mice depleted of glycoprotein VI on platelets or with addition of a Syk inhibitor. We reproduced these results in human blood with combined inhibition of PAR1, PAR4, and Syk.

CONCLUSION

Our results demonstrate that standard TEG is not sensitive to platelet signaling pathways critical for integrin inside-out activation and platelet hemostatic function. Furthermore, we provide the first evidence that PARs and glycoprotein VI play redundant roles in platelet-mediated clot contraction in TEG.

RhoGAP6 interacts with COPI to regulate protein transport

RhoGAP6 is the most highly expressed GTPase-activating protein (GAP) in platelets specific for RhoA. Structurally RhoGAP6 contains a central catalytic GAP domain surrounded by large, disordered N- and C-termini of unknown function. Sequence analysis revealed three conserved consecutive overlapping di-tryptophan motifs close to the RhoGAP6 C-terminus which were predicted to bind to the mu homology domain (MHD) of δ-COP, a component of the COPI vesicle complex. We confirmed an endogenous interaction between RhoGAP6 and δ-COP in human platelets using GST-CD2AP which binds an N-terminal RhoGAP6 SH3 binding motif. Next, we confirmed that the MHD of δ-COP and the di-tryptophan motifs of RhoGAP6 mediate the interaction between both proteins. Each of the three di-tryptophan motifs appeared necessary for stable δ-COP binding. Proteomic analysis of other potential RhoGAP6 di-tryptophan motif binding partners indicated that the RhoGAP6/δ-COP interaction connects RhoGAP6 to the whole COPI complex. 14-3-3 was also established as a RhoGAP6 binding partner and its binding site was mapped to serine 37. We provide evidence of potential cross-regulation between 14-3-3 and δ-COP binding, however, neither δ-COP nor 14-3-3 binding to RhoGAP6 impacted RhoA activity. Instead, analysis of protein transport through the secretory pathway demonstrated that RhoGAP6/δ-COP binding increased protein transport to the plasma membrane, as did a catalytically inactive mutant of RhoGAP6. Overall, we have identified a novel interaction between RhoGAP6 and δ-COP which is mediated by conserved C-terminal di-tryptophan motifs, and which might control protein transport in platelets.

The platelet transcriptome and proteome in Alzheimer's disease and aging: an exploratory cross-sectional study

Introduction: Alzheimer's disease (AD) and aging are associated with platelet hyperactivity. However, the mechanisms underlying abnormal platelet function in AD and aging are yet poorly understood. Methods: To explore the molecular profile of AD and aged platelets, we investigated platelet activation (i.e., CD62P expression), proteome and transcriptome in AD patients, non-demented elderly, and young individuals as controls. Results: AD, aged and young individuals showed similar levels of platelet activation based on CD62P expression. However, AD and aged individuals had a proteomic signature suggestive of increased platelet activation compared with young controls. Transcriptomic profiling suggested the dysregulation of proteolytic machinery involved in regulating platelet function, particularly the ubiquitin-proteasome system in AD and autophagy in aging. The functional implication of these transcriptomic alterations remains unclear and requires further investigation. Discussion: Our data strengthen the evidence of enhanced platelet activation in aging and provide a first glimpse of the platelet transcriptomic changes occurring in AD.

Platelet formation and activation are influenced by neuronal guidance proteins

Platelets are anucleate blood cells derived from megakaryocytes. They link the fundamental functions of hemostasis, inflammation and host defense. They undergo intracellular calcium flux, negatively charged phospholipid translocation, granule release and shape change to adhere to collagen, fibrin and each other, forming aggregates, which are key to several of their functions. In all these dynamic processes, the cytoskeleton plays a crucial role. Neuronal guidance proteins (NGPs) form attractive and repulsive signals to drive neuronal axon navigation and thus refine neuronal circuits. By binding to their target receptors, NGPs rearrange the cytoskeleton to mediate neuron motility. In recent decades, evidence has indicated that NGPs perform important immunomodulatory functions and influence platelet function. In this review, we highlight the roles of NGPs in platelet formation and activation.

Tau aggregates improve the purinergic receptor P2Y12-associated podosome rearrangements in microglial cells

Alzheimer's disease (AD) is a progressive neurodegenerative disease that is associated with protein misfolding, plaque accumulation, neuronal dysfunction, synaptic loss, and cognitive decline. The pathological cascade of AD includes the intracellular Tau hyperphosphorylation and its subsequent aggregation, extracellular Amyloid-β plaque formation and microglia-mediated neuroinflammation. The extracellular release of aggregated Tau is sensed by surveilling microglia through the involvement of various cell surface receptors. Among all, purinergic P2Y12R signaling is involved in microglial chemotaxis towards the damaged neurons. Microglial migration is highly linked with membrane-associated actin remodeling leading to the phagocytosis of extracellular Tau species. Here, we studied the formation of various actin structures such as podosome, lamellipodia and filopodia, in response to extracellular Tau monomers and aggregates. Microglial podosomes are colocalized with actin nucleator protein WASP, Arp2 and TKS5 adaptor protein during Tau-mediated migration. Moreover, the P2Y12 receptors were associated with F-actin-rich podosome structures, which signify the potential of Tau aggregates in microglial chemotaxis through the involvement of actin remodeling.

Dual thick and thin filament linked regulation of stretch- and L-NAME-induced tone in young and senescent murine basilar artery

Stretch-induced vascular tone is an important element of autoregulatory adaptation of cerebral vasculature to maintain cerebral flow constant despite changes in perfusion pressure. Little is known as to the regulation of tone in senescent basilar arteries. We tested the hypothesis, that thin filament mechanisms in addition to smooth muscle myosin-II regulatory-light-chain-(MLC20)-phosphorylation and non-muscle-myosin-II, contribute to regulation of stretch-induced tone. In young BAs (y-BAs) mechanical stretch does not lead to spontaneous tone generation. Stretch-induced tone in y-BAs appeared only after inhibition of NO-release by L-NAME and was fully prevented by treatment with 3 μmol/L RhoA-kinase (ROK) inhibitor Y27632. L-NAME-induced tone was reduced in y-BAs from heterozygous mice carrying a point mutation of the targeting-subunit of the myosin phosphatase, MYPT1 at threonine696 (MYPT1-T696A/+). In y-BAs, MYPT1-T696A-mutation also blunted the ability of L-NAME to increase MLC20-phosphorylation. In contrast, senescent BAs (s-BAs; &gt;24 months) developed stable spontaneous stretch-induced tone and pharmacological inhibition of NO-release by L-NAME led to an additive effect. In s-BAs the MYPT1-T696A mutation also blunted MLC20-phosphorylation, but did not prevent development of stretch-induced tone. In s-BAs from both lines, Y27632 completely abolished stretch- and L-NAME-induced tone. In s-BAs phosphorylation of non-muscle-myosin-S1943 and PAK1-T423, shown to be down-stream effectors of ROK was also reduced by Y27632 treatment. Stretch- and L-NAME tone were inhibited by inhibition of non-muscle myosin (NM-myosin) by blebbistatin. We also tested whether the substrate of PAK1 the thin-filament associated protein, caldesmon is involved in the regulation of stretch-induced tone in advanced age. BAs obtained from heterozygotes Cald1+/− mice generated stretch-induced tone already at an age of 20–21 months old BAs (o-BA). The magnitude of stretch-induced tone in Cald1+/− o-BAs was similar to that in s-BA. In addition, truncation of caldesmon myosin binding Exon2 (CaD-▵Ex2−/−) did not accelerate stretch-induced tone. Our study indicates that in senescent cerebral vessels, mechanisms distinct from MLC20 phosphorylation contribute to regulation of tone in the absence of a contractile agonist. While in y-and o-BA the canonical pathways, i.e., inhibition of MLCP by ROK and increase in pMLC20, predominate, tone regulation in senescence involves ROK regulated mechanisms, involving non-muscle-myosin and thin filament linked mechanisms involving caldesmon.

CDC42 Might Be a Molecular Signature of DWI-FLAIR Mismatch in a Nonhuman Primate Stroke Model

No definitive blood markers of DWI-FLAIR mismatch, a pivotal indicator of salvageable ischemic penumbra brain tissue, are known. We previously reported that CDC42 and RHOA are associated with the ischemic penumbra. Here, we investigated whether plasma CDC42 and RHOA are surrogate markers of DWI-FLAIR mismatch. Sixteen cynomolgus macaques (3 as controls and 13 for the stroke model) were included. Guided by digital subtraction angiography (DSA), a middle cerebral artery occlusion (MCAO) model was established by occluding the middle cerebral artery (MCA) with a balloon. MRI and neurological deficit scoring were performed to evaluate postinfarction changes. Plasma CDC42 and RHOA levels were measured by enzyme-linked immunosorbent assay (ELISA). The stroke model was successfully established in eight monkeys. Based on postinfarction MRI images, experimental animals were divided into a FLAIR (-) group (N = 4) and a FLAIR (+) group (N = 4). Plasma CDC42 in the FLAIR (-) group showed a significant decrease compared with that in the FLAIR (+) group (p < 0.05). No statistically significant difference was observed for plasma RHOA. The FLAIR (-) group showed a milder neurological function deficit and a smaller infarct volume than the FLAIR (+) group (p < 0.05). Therefore, plasma CDC42 might be a new surrogate marker for DWI-FLAIR mismatch.

Gelsolin Modulates Platelet Dense Granule Secretion and Hemostasis via the Actin Cytoskeleton

BACKGROUND AND OBJECTIVE

 The mechanisms underlying platelet granule release are not fully understood. The actin cytoskeleton serves as the platelet's structural framework that is remodeled upon platelet activation. Gelsolin is a calcium-dependent protein that severs and caps existing actin filaments although its role in modulating platelet granule exocytosis is unknown.

METHODS

 The hemostatic function of wild-type (WT) and gelsolin null (Gsn-/- ) mice was measured ex vivo by rotational thromboelastometry analysis of whole blood. Platelets were purified from WT and Gsn-/- mouse blood and activated with thrombin. Platelet aggregation was assessed by light-transmission aggregometry. Clot retraction was measured to assess outside-in integrin signaling. Adenosine triphosphate (ATP) release and surface P-selectin were measured as markers of dense- and α-granule secretion, respectively.

RESULTS

 The kinetics of agonist-induced aggregation, clot retraction, and ATP release were accelerated in Gsn-/- platelets relative to WT. However, levels of surface P-selectin were diminished in Gsn-/- platelets. ATP release was also accelerated in WT platelets pretreated with the actin-depolymerizing drug cytochalasin D, thus mimicking the kinetics observed in Gsn-/- platelets. Conversely, ATP release kinetics were normalized in Gsn-/- platelets treated with the actin polymerization agonist jasplakinolide. Rab27b and Munc13-4 are vesicle-priming proteins known to promote dense granule secretion. Co-immunoprecipitation indicates that the association between Rab27b and Munc13-4 is enhanced in Gsn-/- platelets.

CONCLUSIONS

 Gelsolin regulates the kinetics of hemostasis by modulating the platelet's actin cytoskeleton and the protein machinery of dense granule exocytosis.

Platelet response to influenza vaccination reflects effects of aging

Platelets are uniquely positioned as mediators of not only hemostasis but also innate immunity. However, how age and geriatric conditions such as frailty influence platelet function during an immune response remains unclear. We assessed the platelet transcriptome at baseline and following influenza vaccination in Younger (age 21-35) and Older (age ≥65) adults (including community-dwelling individuals who were largely non-frail and skilled nursing facility (SNF)-resident adults who nearly all met criteria for frailty). Prior to vaccination, we observed an age-associated increase in the expression of platelet activation and mitochondrial RNAs and decrease in RNAs encoding proteins mediating translation. Age-associated differences were also identified in post-vaccination response trajectories over 28 days. Using tensor decomposition analysis, we found increasing RNA expression of genes in platelet activation pathways in young participants, but decreasing levels in (SNF)-resident adults. Translation RNA trajectories were inversely correlated with these activation pathways. Enhanced platelet activation was found in community-dwelling older adults at the protein level, compared to young individuals both prior to and post-vaccination; whereas SNF residents showed decreased platelet activation compared to community-dwelling older adults that could reflect the influence of decreased translation RNA expression. Our results reveal alterations in the platelet transcriptome and activation responses that may contribute to age-associated chronic inflammation and the increased incidence of thrombotic and pro-inflammatory diseases in older adults.

Rho GTPase Signaling in Platelet Regulation and Implication for Antiplatelet Therapies

Platelets play a vital role in regulating hemostasis and thrombosis. Rho GTPases are well known as molecular switches that control various cellular functions via a balanced GTP-binding/GTP-hydrolysis cycle and signaling cascade through downstream effectors. In platelets, Rho GTPases function as critical regulators by mediating signal transduction that drives platelet activation and aggregation. Mostly by gene targeting and pharmacological inhibition approaches, Rho GTPase family members RhoA, Rac1, and Cdc42 have been shown to be indispensable in regulating the actin cytoskeleton dynamics in platelets, affecting platelet shape change, spreading, secretion, and aggregation, leading to thrombus formation. Additionally, studies of Rho GTPase function using platelets as a non-transformed model due to their anucleated nature have revealed valuable information on cell signaling principles. This review provides an updated summary of recent advances in Rho GTPase signaling in platelet regulation. We also highlight pharmacological approaches that effectively inhibited platelet activation to explore their possible development into future antiplatelet therapies.

Filamin A in platelets: Bridging the (signaling) gap between the plasma membrane and the actin cytoskeleton

Platelets are anucleate cells that are essential for hemostasis and wound healing. Upon activation of the cell surface receptors by their corresponding extracellular ligands, platelets undergo rapid shape change driven by the actin cytoskeleton; this shape change reaction is modulated by a diverse array of actin-binding proteins. One actin-binding protein, filamin A (FLNA), cross-links and stabilizes subcortical actin filaments thus providing stability to the cell membrane. In addition, FLNA binds the intracellular portion of multiple cell surface receptors and acts as a critical intracellular signaling scaffold that integrates signals between the platelet's plasma membrane and the actin cytoskeleton. This mini-review summarizes how FLNA transduces critical cell signals to the platelet cytoskeleton.

Targeting TLR2/Rac1/cdc42/JNK Pathway to Reveal That Ruxolitinib Promotes Thrombocytopoiesis

BACKGROUND

Thrombocytopenia has long been considered an important complication of chemotherapy and radiotherapy, which severely limits the effectiveness of cancer treatment and the overall survival of patients. However, clinical treatment options are extremely limited so far. Ruxolitinib is a potential candidate.

METHODS

The impact of ruxolitinib on the differentiation and maturation of K562 and Meg-01 cells megakaryocytes (MKs) was examined by flow cytometry, Giemsa and Phalloidin staining. A mouse model of radiation-injured thrombocytopenia (RIT) was employed to evaluate the action of ruxolitinib on thrombocytopoiesis. Network pharmacology, molecular docking, drug affinity responsive target stability assay (DARTS), RNA sequencing, protein blotting and immunofluorescence analysis were applied to explore the targets and mechanisms of action of ruxolitinib.

RESULTS

Ruxolitinib can stimulate MK differentiation and maturation in a dose-dependent manner and accelerates recovery of MKs and thrombocytopoiesis in RIT mice. Biological targeting analysis showed that ruxolitinib binds directly to Toll Like Receptor 2 (TLR2) to activate Rac1/cdc42/JNK, and this action was shown to be blocked by C29, a specific inhibitor of TLR2.

CONCLUSIONS

Ruxolitinib was first identified to facilitate MK differentiation and thrombocytopoiesis, which may alleviate RIT. The potential mechanism of ruxolitinib was to promote MK differentiation via activating the Rac1/cdc42/JNK pathway through binding to TLR2.

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

Genetic Polymorphisms Associated with Prothrombin Time and Activated Partial Thromboplastin Time in Chinese Healthy Population

(1) Background: The purpose of this study was to evaluate the effect of gene polymorphisms on prothrombin time (PT) and activated partial thromboplastin time (APTT) in a healthy Chinese population. (2) Methods: A total of 403 healthy volunteers from a series of novel oral anticoagulants (NOACs) bioequivalence trials in China were included. Coagulation tests for PT and APTT were performed in the central lab at Peking University First Hospital. Whole-exome sequencing (WES) and genome-wide association analysis were performed. (3) Results: In the correlation analysis of PT, 105 SNPs from 84 genes reached the genome-wide significance threshold (p < 1 × 10−5). Zinc Finger Protein 594 (ZNF594) rs184838268 (p = 4.50 × 10−19) was most significantly related to PT, and Actinin Alpha 1 (ACTN1) was found to interact most with other candidate genes. Significant associations with previously reported candidate genes Aurora Kinase B (AURKB), Complement C5(C5), Clock Circadian Regulator (CLOCK), and Histone Deacetylase 9(HDAC9) were detected in our dataset (p < 1 × 10−5). PiggyBac Transposable Element Derived 2(PGBD2) rs75935520 (p = 4.49 × 10−6), Bromodomain Adjacent To Zinc Finger Domain 2A(BAZ2A) rs199970765 (p = 5.69 × 10−6) and Protogenin (PRTG) rs80064850 (p = 8.69 × 10−6) were significantly correlated with APTT (p < 1 × 10−5). The heritability values of PT and APTT were 0.83 and 0.64, respectively; (4) Conclusion: The PT and APTT of healthy populations are affected by genetic polymorphisms. ZNF594 and ACTN1 variants could be novel genetic markers of PT, while PRTG polymorphisms might be associated with APTT levels. The findings could be attributed to ethnic differences, and need further investigation.

Neonatal Platelets: Lower G12/13 Expression Contributes to Reduced Secretion of Dense Granules

Despite fully functional primary hemostasis, platelets of healthy neonates exhibit hypoaggregability and secretion defects, which may be adaptations to specific requirements in this developmental stage. The etiologies for reduced signal transduction vary with the type of agonist. The discovered peculiarities are lower receptor densities, reduced calcium mobilization, and functional impairments of G proteins. Reduced secretion of dense granules has been attributed to lower numbers of granules. Signaling studies with adult platelets have shown a regulating effect of the G_12/13 signaling pathway on dense granule secretion via RhoA. We comparatively analyzed secretion profiles using flow cytometry and expression levels of G_q, G_i, and G_12/13 using Western blot analysis in platelets from cord blood and adults. Furthermore, we evaluated Rho activation after in vitro platelet stimulation with thrombin using a pulldown assay. We observed a markedly reduced expression of the dense granule marker CD63 on neonatal platelets after thrombin stimulation. G_α12/13 expression was significantly decreased in neonatal platelets and correlated with lower Rho activation after thrombin stimulation. We conclude that lower expression of G_12/13 in neonatal platelets results in attenuated activation of Rho and may contribute to reduced secretion of dense granules after exposure to thrombin.

Non-canonical Sonic Hedgehog signaling amplifies platelet reactivity and thrombogenicity

Sonic Hedgehog signaling amplifies platelet activation.

Targeting Shh signaling attenuates hemostasis and thrombosis.

Sonic Hedgehog (Shh) is a morphogen in vertebrate embryos that is also associated with organ homeostasis in adults. We report here that human platelets, though enucleate, synthesize Shh from preexisting mRNAs upon agonist stimulation, and mobilize it for surface expression and release on extracellular vesicles, thus alluding to its putative role in platelet activation. Shh, in turn, induced a wave of noncanonical signaling in platelets leading to activation of small GTPase Ras homolog family member A and phosphorylation of myosin light chain in activated protein kinase-dependent manner. Remarkably, agonist-induced thrombogenic responses in platelets, which include platelet aggregation, granule secretion, and spreading on immobilized fibrinogen, were significantly attenuated by inhibition of Hedgehog signaling, thus, implicating inputs from Shh in potentiation of agonist-mediated platelet activation. In consistence, inhibition of the Shh pathway significantly impaired arterial thrombosis in mice. Taken together, the above observations strongly support a feed-forward loop of platelet stimulation triggered locally by Shh, similar to ADP and thromboxane A2, that contributes significantly to the stability of occlusive arterial thrombus and that can be investigated as a potential therapeutic target in thrombotic disorders.

Longitudinal Variations of CDC42 in Patients With Acute Ischemic Stroke During 3-Year Period: Correlation With CD4+ T Cells, Disease Severity, and Prognosis

Objective

Cell division cycle 42 (CDC42) modulates CD4⁺ T-cell differentiation, blood lipids, and neuronal apoptosis and is involved in the pathogenesis of acute ischemic stroke (AIS); however, the clinical role of CDC42 in AIS remains unanswered. This study aimed to evaluate the expression of CDC42 in a 3-year follow-up and its correlation with disease severity, T helper (Th)1/2/17 cells, and the prognosis in patients with AIS.

Methods

Blood CDC42 was detected in 143 patients with AIS at multiple time points during the 3-year follow-up period and in 70 controls at admission by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). In addition, blood Th1, Th2, and Th17 cells and their secreted cytokines (interferon-γ (IFN-γ), interleukin-4 (IL-4), and interleukin-17A (IL-17A)) in patients with AIS were detected by flow cytometry and enzyme-linked immunosorbent assay (ELISA), respectively.

Results

Compared with controls (p < 0.001), CDC42 was reduced in patients with AIS. CDC42 was negatively correlated with the National Institutes of Health Stroke Scale (NIHSS) score (p < 0.001), whereas, in patients with AIS (all p < 0.050), it was positively associated with Th2 cells and IL-4 but negatively correlated with Th17 cells and IL-17A. CDC42 was decreased from admission to 3 days and gradually increased from 3 days to 3 years in patients with AIS (P<0.001). In a 3-year follow-up, 24 patients with AIS recurred and 8 patients died. On the 3rd day, 7th day, 1st month, 3rd month, 6th month, 1st year, 2nd year, and 3rd year, CDC42 was decreased in recurrent patients than that in non-recurrent patients (all p < 0.050). CDC42 at 7 days (p = 0.033) and 3 months (p = 0.023) was declined in reported deceased patients than in survived patients.

Conclusion

CDC42 is used as a biomarker to constantly monitor disease progression and recurrence risk of patients with AIS.

Activation mechanism dependent surface exposure of cellular factor XIII on activated platelets and platelet microparticles

BACKGROUND

Platelets contain a high amount of potentially active A subunit dimer of coagulation factor XIII (cellular FXIII; cFXIII). It is of cytoplasmic localization, not secreted, but becomes translocated to the surface of platelets activated by convulxin and thrombin (CVX+Thr).

OBJECTIVE

To explore the difference in cFXIII translocation between receptor mediated and non-receptor mediated platelet activation and if translocation can also be detected on platelet-derived microparticles. Our aim was also to shed some light on the mechanism of cFXIII translocation.

METHODS

Gel-filtered platelets were activated by CVX+Thr or Ca2+ -ionophore (calcimycin). The translocation of cFXIII and phosphatidylserine (PS) to the surface of activated platelets and platelet-derived microparticles was investigated by flow cytometry, immunofluorescence, and immune electron microscopy. Fluo-4-AM fluorescence was used for the measurement of intracellular Ca2+ concentration.

RESULTS

Receptor mediated activation by CVX+Thr exposed cFXIII to the surface of more than 60% of platelets. Electron microscopy revealed microparticles with preserved membrane structure and microparticles devoid of labeling for membrane glycoprotein CD41a. cFXIII was observed on both types of microparticles but was more abundant in the absence of CD41a. Rhosin, a RhoA inhibitor, significantly decreased cFXIII translocation. Non-receptor mediated activation of platelets by calcimycin elevated intracellular Ca2+ concentration, induced the translocation of PS to the surface of platelets and microparticles, but failed to expose cFXIII.

CONCLUSIONS

The elevation of intracellular Ca2+ concentration is sufficient for the translocation of PS from the internal layer of the membrane, while the translocation of cFXIII from the platelet cytoplasm requires additional receptor mediated mechanism(s).

The voltage-gated K+ channel Kv1.3 modulates platelet motility and α2β1 integrin-dependent adhesion to collagen

Kv1.3 is a voltage-gated K+-selective channel with roles in immunity, insulin-sensitivity, neuronal excitability and olfaction. Despite being one of the largest ionic conductances of the platelet surface membrane, its contribution to platelet function is poorly understood. Here we show that Kv1.3-deficient platelets display enhanced ADP-evoked platelet aggregation and secretion, and an increased surface expression of platelet integrin αIIb. In contrast, platelet adhesion and thrombus formation in vitro under arterial shear conditions on surfaces coated with collagen were reduced for samples from Kv1.3-/- compared to wild type mice. Use of collagen-mimetic peptides revealed a specific defect in the engagement with α2β1. Kv1.3-/- platelets developed significantly fewer, and shorter, filopodia than wild type platelets during adhesion to collagen fibrils. Kv1.3-/- mice displayed no significant difference in thrombus formation within cremaster muscle arterioles using a laser-induced injury model, thus other pro-thrombotic pathways compensate in vivo for the adhesion defect observed in vitro. This may include the increased platelet counts of Kv1.3-/- mice, due in part to a prolonged lifespan. The ability of Kv1.3 to modulate integrin-dependent platelet adhesion has important implications for understanding its contribution to normal physiological platelet function in addition to its reported roles in auto-immune diseases and thromboinflammatory models of stroke.

Turning Platelets Off and On: Role of RhoGAPs and RhoGEFs in Platelet Activity

Platelet cytoskeletal reorganisation is a critical component of platelet activation and thrombus formation in haemostasis. The Rho GTPases RhoA, Rac1 and Cdc42 are the primary drivers in the dynamic reorganisation process, leading to the development of filopodia and lamellipodia which dramatically increase platelet surface area upon activation. Rho GTPases cycle between their active (GTP-bound) and inactive (GDP-bound) states through tightly regulated processes, central to which are the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs catalyse the dissociation of GDP by inducing changes in the nucleotide binding site, facilitating GTP binding and activating Rho GTPases. By contrast, while all GTPases possess intrinsic hydrolysing activity, this reaction is extremely slow. Therefore, GAPs catalyse the hydrolysis of GTP to GDP, reverting Rho GTPases to their inactive state. Our current knowledge of these proteins is constantly being updated but there is considerably less known about the functionality of Rho GTPase specific GAPs and GEFs in platelets. In the present review, we discuss GAP and GEF proteins for Rho GTPases identified in platelets, their regulation, biological function and present a case for their further study in platelets.

PAR4-Mediated PI3K/Akt and RhoA/ROCK Signaling Pathways Are Essential for Thrombin-Induced Morphological Changes in MEG-01 Cells

Thrombin stimulates platelets via a dual receptor system of protease-activated receptors (PARs): PAR1 and PAR4. PAR1 activation induces a rapid and transient signal associated with the initiation of platelet aggregation, whereas PAR4 activation results in a prolonged signal, required for later phases, that regulates the stable formation of thrombus. In this study, we observed differential signaling pathways for thrombin-induced PAR1 and PAR4 activation in a human megakaryoblastic leukemia cell line, MEG-01. Interestingly, thrombin induced both calcium signaling and morphological changes in MEG-01 cells via the activation of PAR1 and PAR4, and these intracellular events were very similar to those observed in platelets shown in previous studies. We developed a novel image-based assay to quantitatively measure the morphological changes in living cells, and observed the underlying mechanism for PAR1- and PAR4-mediated morphological changes in MEG-01 cells. Selective inhibition of PAR1 and PAR4 by vorapaxar and BMS-986120, respectively, showed that thrombin-induced morphological changes were primarily mediated by PAR4 activation. Treatment of a set of kinase inhibitors and 2-aminoethoxydiphenyl borate (2-APB) revealed that thrombin-mediated morphological changes were primarily regulated by calcium-independent pathways and PAR4 activation-induced PI3K/Akt and RhoA/ROCK signaling pathways in MEG-01 cells. These results indicate the importance of PAR4-mediated signaling pathways in thrombin-induced morphological changes in MEG-01 cells and provide a useful in vitro cellular model for platelet research.

Paxillin binding to the PH domain of kindlin-3 in platelets is required to support integrin αIIbβ3 outside-in signaling

BACKGROUND

Kindlin-3 is essential for supporting the bidirectional signaling of integrin αIIbβ3 in platelets by bridging the crosstalk between integrin αIIbβ3 and the cytoplasmic signaling adaptors.

OBJECTIVE

In this study, we identified a previously unrecognized paxillin binding site in the pleckstrin homology (PH) domain of kindlin-3 and verified its functional significance.

METHODS

Structure-based approaches were employed to identify the paxillin binding site in the PH domain of kindlin-3. In addition, the bidirectional signaling of integrin αIIbβ3 were evaluated in both human and mouse platelets.

RESULTS

In brief, we found that a β1-β2 loop in the PH domain of kindlin-3, an important part of the canonical membrane phospholipid binding pocket, was also involved in mediating paxillin interaction. Interestingly, the binding sites of paxillin and membrane phospholipids in the PH domain of kindlin-3 were mutually exclusive. Specific disruption of paxillin binding to the PH domain by point mutations inhibited platelet spreading on immobilized fibrinogen while having no inhibition on soluble fibrinogen binding to stimulated platelets. In addition, a membrane-permeable peptide derived from the β1-β2 loop in the PH domain of kindlin-3 was capable of inhibiting platelet spreading and clot retraction, but it had no effect on soluble fibrinogen binding to platelets and platelet aggregation. Treatment with this peptide significantly reduced thrombus formation in mice.

CONCLUSION

Taken together, these findings suggest that interaction between paxillin and the PH domain of kindlin-3 plays an important role in supporting integrin αIIbβ3 outside-in signaling in platelets, thus providing a novel antithrombotic target.

The Proteoglycan Biglycan Modulates Platelet Adhesion and Thrombus Formation in a GPVI-Dependent Manner

BACKGROUND

Vascular injury induces the exposure of subendothelial extracellular matrix (ECM) important to serve as substrate for platelets to adhere to the injured vessel wall to avoid massive blood loss. Different ECM proteins are known to initiate platelet adhesion and activation. In atherosclerotic mice, the small, leucine-rich proteoglycan biglycan is important for the regulation of thrombin activity via heparin cofactor II. However, nothing is known about the role of biglycan for hemostasis and thrombosis under nonatherosclerotic conditions.

METHODS

The role of biglycan for platelet adhesion and thrombus formation was investigated using a recombinant protein and biglycan knockout mice.

RESULTS

The present study identified biglycan as important ECM protein for the adhesion and activation of platelets, and the formation of three-dimensional thrombi under flow conditions. Platelet adhesion to immobilized biglycan induces the reorganization of the platelet cytoskeleton. Mechanistically, biglycan binds and activates the major collagen receptor glycoprotein (GP)VI, because reduced platelet adhesion to recombinant biglycan was observed when GPVI was blocked and enhanced tyrosine phosphorylation in a GPVI-dependent manner was observed when platelets were stimulated with biglycan. In vivo, the deficiency of biglycan resulted in reduced platelet adhesion to the injured carotid artery and prolonged bleeding times.

CONCLUSIONS

Loss of biglycan in the vessel wall of mice but not in platelets led to reduced platelet adhesion at the injured carotid artery and prolonged bleeding times, suggesting a crucial role for biglycan as ECM protein that binds and activates platelets via GPVI upon vessel injury.

Thiol Isomerases Orchestrate Thrombosis and Hemostasis

Significance: Since protein disulfide isomerase (PDI) was first described in 1963, researchers have shown conclusively that PDI and sibling proteins are quintessential for thrombus formation. PDI, endoplasmic reticulum protein (ERp)5, ERp57, and ERp72 are released from platelets and vascular cells and interact with integrin αIIbβ3 on the outer surface of platelets. Recent Advances: At the cell surface they influence protein folding and function, propagating thrombosis and maintaining hemostasis. TMX1, which is a transmembrane thiol isomerase, is the first family member shown to negatively regulate platelets. Targets of thiol isomerases have been identified, including integrin α2β1, Von Willebrand Factor, GpIbα, nicotinamide adenine dinucleotide phosphate oxidase (Nox)-1, Nox-2, and tissue factor, all of which are pro-thrombotic, and several of which are on the cell surface. In spite of this, PDI can paradoxically catalyze the delivery of nitric oxide to platelets, which decrease thrombus formation. Critical Issues: Although the overall effect of PDI is to positively regulate platelet activation, it is still unclear how thiol isomerases function in pro-thrombotic states, such as obesity, diabetes, and cancer. In parallel, there has been a surge in the development of novel thiol isomerase inhibitors, which display selectivity, potency and modulate thrombosis and hemostasis. The availability of selective thiol isomerase inhibitors has culminated in clinical trials, with promising outcomes for the prevention of cancer-associated thrombosis. Future Directions: Altogether, thiol isomerases are perceived as an orchestrating force that regulates thrombus development. In the current review, we will explore the history of PDI in cardiovascular biology, detail known mechanisms of action, and summarize known thiol isomerase inhibitors.

Mice lacking PECAM-1 and Ceacam1 have an aberrant platelet and thrombus phenotype

The immunoglobulin (Ig)-immunoreceptor tyrosine-based inhibitory motif (ITIM) bearing receptors, PECAM-1 and CEACAM1 have been shown net negative regulators of platelet-collagen interactions and hemi-ITAM signalling pathways. In this study, a double knockout (DKO) mouse was developed with deleted PECAM-1 and CEACAM1 to study their combined contribution in platelet activation by glycoprotein VI, C-type lectin-like receptor 2 (CLEC-2), protease activated receptor PAR-4, ADP purinergic receptors and thromboxane receptor TP A2 pathways. Additionally, their collective contribution was examined in thrombus formation under high shear and microvascular thrombosis using in vivo models. DKO platelets responded normally to ADP purinergic receptors and TP A2 pathway. However, DKO platelets released significantly higher amounts of P-selectin compared to hyper-responsive Pecam-1-/- or Ceacam1-/- versus wild-type (WT) upon stimulation with collagen related peptide or rhodocytin. Contrastingly, DKO platelets released increased amounts of P-selectin upon stimulation with PAR-4 agonist peptide or thrombin but not Pecam-1-/-, Ceacam1-/- or WT platelets. Blockade of phospholipase C (PLC) or Rho A kinase revealed that DKO platelets enhanced alpha granule release via PAR-4/Gαq/PLC signalling without crosstalk with Src/Syk or G12/13 signalling pathways. This DKO model showed a significant increase in thrombus formation compared to the hyper-responsive Ceacam1-/- or Pecam-1-/- versus WT phenotype. DKO platelets have similar glycoprotein surface expression compared to Pecam-1-/-, Ceacam1-/- and WT platelets. PECAM-1 and CEACAM1 work in concert to negatively regulate hemiITAM signalling, platelet-collagen interactions and PAR-4 Gαq protein coupled signalling pathways. Both PECAM-1 and CEACAM1 are required for negative regulation of platelet activation and microvascular thrombosis in vivo.

Platelets: a potential role in chronic respiratory diseases?

Platelets are small anucleate cells known for their role in haemostasis and thrombosis. In recent years, an increasing number of observations have suggested that platelets are also immune cells and key modulators of immunity. They express different receptors and molecules that allow them to respond to pathogens, and to interact with other immune cells. Platelets were linked to the pathogenesis of some inflammatory disorders including respiratory diseases such as asthma and idiopathic pulmonary fibrosis. Here, we discuss the involvement of platelets in different immune responses, and we focus on their potential role in various chronic lung diseases.

In addition to their essential role in haemostasis and thrombosis, platelets are strong modulators of different immune responses, and could be involved in the physiopathology of several chronic airway diseaseshttps://bit.ly/3cB6Xnj

Platelets' RNA as biomarker trove for differentiation of early-stage hepatocellular carcinoma from underlying cirrhotic nodules

BACKGROUND & AIMS

Among the multiplicity of factors involved in rising incidence of hepatocellular carcinoma (HCC)-the second deadliest cancer, late diagnosis of early-stage HCC nodules originating from late-stage cirrhotic nodules is the most crucial. In recent years, Tumor-educated platelets (TEPs) have emerged as a strong multimodal tool to be used in liquid-biopsy of cancers because of changes in their mRNA content. This study assessed the reliability of selected mRNA repertoire of platelets as biomarkers to differentiate early HCC from late-stage cirrhotic nodules.

METHODS

Quantitative real time PCR (qRT-PCR) was used to evaluate expression levels of selected platelets-specific mRNA between HCC patients compared to cirrhosis patients. ROC curve analysis assessed the sensitivity and specificity of the biomarkers.

RESULTS

RhoA, CTNNB1 and SPINK1 showed a significant 3.3-, 3.2- and 3.18-folds upregulation, respectively, in HCC patients compared to cirrhosis patients while IFITM3 and SERPIND1 presented a 2.24-fold change. Strikingly, CD41+ platelets also demonstrated a marked difference of expression in HCC and cirrhosis groups.

CONCLUSIONS

Our study reports liquid biopsy-based platelets mRNA signature for early diagnosis of HCC from underlying cirrhotic nodules. Moreover, differential expression of CD41+ platelets in two groups provides new insights into a probable link between CD41 expression on platelets with the progression of cirrhosis to HCC.

Knockdown of PAK1 Inhibits the Proliferation and Invasion of Non-Small Cell Lung Cancer Cells Through the ERK Pathway

The p21-activated kinase (PAK) family of serine/threonine kinases plays a pivotal role in various human tumors, as supported by our previous report on the overexpressed PAK isoforms in non-small cell lung cancer (NSCLC). To better understand the role of PAKs in tumorigenesis, the authors examined PAK1 expression patterns and its significance in NSCLC. It was demonstrated by immunohistochemical staining that PAK1 was increased and localized in the cytoplasm in 151 of 207 cases. High levels of PAK1 expression correlated with a histologic type of tumor (squamous cell carcinoma), tumor node metastasis stage, and lymph nodal status. We also examined the biological role of PAK1 in lung cancer cell lines transfected with PAK1-small interfering RNA. Decreased expression of PAK1 inhibited lung cancer cell proliferation and invasion, which is the major cause of lung cancer malignancy. Downregulated expression of PAK1 hampered rapidly accelerated fibrosarcoma/mitogen-activated extracellular signal-regulated kinase/extracellular signal-regulated kinase pathway activity but did not affect Wnt/β-catenin signaling. Our findings suggest that PAK1 is an important oncogene in NSCLC, as decreased expression of PAK1 inhibited the proliferation and invasion of NSCLC cells by blocking the ERK pathway. These results provide evidence for using PAK1 inhibition as potential anticancer therapy.

DIAPH1 Variants in Non-East Asian Patients With Sporadic Moyamoya Disease

Importance

Moyamoya disease (MMD), a progressive vasculopathy leading to narrowing and ultimate occlusion of the intracranial internal carotid arteries, is a cause of childhood stroke. The cause of MMD is poorly understood, but genetic factors play a role. Several familial forms of MMD have been identified, but the cause of most cases remains elusive, especially among non-East Asian individuals.

Objective

To assess whether ultrarare de novo and rare, damaging transmitted variants with large effect sizes are associated with MMD risk.

Design, Setting, and Participants

A genetic association study was conducted using whole-exome sequencing case-parent MMD trios in a small discovery cohort collected over 3.5 years (2016-2019); data were analyzed in 2020. Medical records from US hospitals spanning a range of 1 month to 1.5 years were reviewed for phenotyping. Exomes from a larger validation cohort were analyzed to identify additional rare, large-effect variants in the top candidate gene. Participants included patients with MMD and, when available, their parents. All participants who met criteria and were presented with the option to join the study agreed to do so; none were excluded. Twenty-four probands (22 trios and 2 singletons) composed the discovery cohort, and 84 probands (29 trios and 55 singletons) composed the validation cohort.

Main Outcomes and Measures

Gene variants were identified and filtered using stringent criteria. Enrichment and case-control tests assessed gene-level variant burden. In silico modeling estimated the probability of variant association with protein structure. Integrative genomics assessed expression patterns of MMD risk genes derived from single-cell RNA sequencing data of human and mouse brain tissue.

Results

Of the 24 patients in the discovery cohort, 14 (58.3%) were men and 18 (75.0%) were of European ancestry. Three of 24 discovery cohort probands contained 2 do novo (1-tailed Poisson P = 1.1 × 10-6) and 1 rare, transmitted damaging variant (12.5% of cases) in DIAPH1 (mammalian diaphanous-1), a key regulator of actin remodeling in vascular cells and platelets. Four additional ultrarare damaging heterozygous DIAPH1 variants (3 unphased) were identified in 3 other patients in an 84-proband validation cohort (73.8% female, 77.4% European). All 6 patients were non-East Asian. Compound heterozygous variants were identified in ena/vasodilator-stimulated phosphoproteinlike protein EVL, a mammalian diaphanous-1 interactor that regulates actin polymerization. DIAPH1 and EVL mutant probands had severe, bilateral MMD associated with transfusion-dependent thrombocytopenia. DIAPH1 and other MMD risk genes are enriched in mural cells of midgestational human brain. The DIAPH1 coexpression network converges in vascular cell actin cytoskeleton regulatory pathways.

Conclusions and Relevance

These findings provide the largest collection to date of non-East Asian individuals with sporadic MMD harboring pathogenic variants in the same gene. The results suggest that DIAPH1 is a novel MMD risk gene and impaired vascular cell actin remodeling in MMD pathogenesis, with diagnostic and therapeutic ramifications.

Whole-exome imputation within UK Biobank powers rare coding variant association and fine-mapping analyses

Exome association studies to date have generally been underpowered to systematically evaluate the phenotypic impact of very rare coding variants. We leveraged extensive haplotype sharing between 49,960 exome-sequenced UK Biobank participants and the remainder of the cohort (total N~500K) to impute exome-wide variants with accuracy (R²>0.5) down to minor allele frequency (MAF) ~0.00005. Association and fine-mapping analyses of 54 quantitative traits identified 1,189 significant associations (P<5 x 10⁻⁸) involving 675 distinct rare protein-altering variants (MAF<0.01) that passed stringent filters for likely causality. Across all traits, 49% of associations (578/1,189) occurred in genes with two or more hits; follow-up analyses of these genes identified allelic series containing up to 45 distinct likely-causal variants. Our results demonstrate the utility of within-cohort imputation in population-scale GWAS cohorts, provide a catalog of likely-causal, large-effect coding variant associations, and foreshadow the insights that will be revealed as genetic biobank studies continue to grow.

Pharmacologic targeting of Cdc42 GTPase by a small molecule Cdc42 activity-specific inhibitor prevents platelet activation and thrombosis

Gene targeting of Cdc42 GTPase has been shown to inhibit platelet activation. In this study, we investigated a hypothesis that inhibition of Cdc42 activity by CASIN, a small molecule Cdc42 Activity-Specific INhibitor, may down regulate platelet activation and thrombus formation. We investigated the effects of CASIN on platelet activation in vitro and thrombosis in vivo. In human platelets, CASIN, but not its inactive analog Pirl7, blocked collagen induced activation of Cdc42 and inhibited phosphorylation of its downstream effector, PAK1/2. Moreover, addition of CASIN to washed human platelets inhibited platelet spreading on immobilized fibrinogen. Treatment of human platelets with CASIN inhibited collagen or thrombin induced: (a) ATP secretion and platelet aggregation; and (b) phosphorylation of Akt, ERK and p38-MAPK. Pre-incubation of platelets with Pirl7, an inactive analog of CASIN, failed to inhibit collagen induced aggregation. Washing of human platelets after incubation with CASIN eliminated its inhibitory effect on collagen induced aggregation. Intraperitoneal administration of CASIN to wild type mice inhibited ex vivo aggregation induced by collagen but did not affect the murine tail bleeding times. CASIN administration, prior to laser-induced injury in murine cremaster muscle arterioles, resulted in formation of smaller and unstable thrombi compared to control mice without CASIN treatment. These data suggest that pharmacologic targeting of Cdc42 by specific and reversible inhibitors may lead to the discovery of novel antithrombotic agents.

miR-204-5p and Platelet Function Regulation: Insight into a Mechanism Mediated by CDC42 and GPIIbIIIa

BACKGROUND

 Several platelet-derived microRNAs are associated with platelet reactivity (PR) and clinical outcome in cardiovascular patients. We previously showed an association between miR-204-5p and PR in stable cardiovascular patients, but data on functional mechanisms are lacking.

AIMS

 To validate miR-204-5p as a regulator of PR in platelet-like structures (PLS) derived from human megakaryocytes and to address mechanistic issues.

METHODS

 Human hematopoietic stem cells were differentiated into megakaryocytes, enabling the transfection of miR-204-5p and the recovery of subsequent PLS. The morphology of transfected megakaryocytes and PLS was characterized using flow cytometry and microscopy. The functional impact of miR-204-5p was assessed using a flow assay, the quantification of the activated form of the GPIIbIIIa receptor, and a fibrinogen-binding assay. Quantitative polymerase chain reaction and western blot were used to evaluate the impact of miR-204-5p on a validated target, CDC42. The impact of CDC42 modulation was investigated using a silencing strategy.

RESULTS

 miR-204-5p transfection induced cytoskeletal changes in megakaryocytes associated with the retracted protrusion of proPLS, but it had no impact on the number of PLS released. Functional assays showed that the PLS produced by megakaryocytes transfected with miR-204-5p were more reactive than controls. This phenotype is mediated by the regulation of GPIIbIIIa expression, a key contributor in platelet-fibrinogen interaction. Similar results were obtained after CDC42 silencing, suggesting that miR-204-5p regulates PR, at least in part, via CDC42 downregulation.

CONCLUSION

 We functionally validated miR-204-5p as a regulator of the PR that occurs through CDC42 downregulation and regulation of fibrinogen receptor expression.

RhoA/Cdc42 signaling drives cytoplasmic maturation but not endomitosis in megakaryocytes

Megakaryocytes (MKs), the precursors of blood platelets, are large, polyploid cells residing mainly in the bone marrow. We have previously shown that balanced signaling of the Rho GTPases RhoA and Cdc42 is critical for correct MK localization at bone marrow sinusoids in vivo. Using conditional RhoA/Cdc42 double-knockout (DKO) mice, we reveal here that RhoA/Cdc42 signaling is dispensable for the process of polyploidization in MKs but essential for cytoplasmic MK maturation. Proplatelet formation is virtually abrogated in the absence of RhoA/Cdc42 and leads to severe macrothrombocytopenia in DKO animals. The MK maturation defect is associated with downregulation of myosin light chain 2 (MLC2) and β1-tubulin, as well as an upregulation of LIM kinase 1 and cofilin-1 at both the mRNA and protein level and can be linked to impaired MKL1/SRF signaling. Our findings demonstrate that MK endomitosis and cytoplasmic maturation are separately regulated processes, and the latter is critically controlled by RhoA/Cdc42.

Assessment of the effects of Syk and BTK inhibitors on GPVI-mediated platelet signaling and function

Spleen tyrosine kinase (Syk) and Bruton's tyrosine kinase (BTK) play critical roles in platelet physiology, facilitating intracellular immunoreceptor tyrosine-based activation motif (ITAM)-mediated signaling downstream of platelet glycoprotein VI (GPVI) and GPIIb/IIIa receptors. Small molecule tyrosine kinase inhibitors (TKIs) targeting Syk and BTK have been developed as antineoplastic and anti-inflammatory therapeutics and have also gained interest as antiplatelet agents. Here, we investigate the effects of 12 different Syk and BTK inhibitors on GPVI-mediated platelet signaling and function. These inhibitors include four Syk inhibitors, Bay 61-3606, R406 (fostamatinib), entospletinib, TAK-659; four irreversible BTK inhibitors, ibrutinib, acalabrutinib, ONO-4059 (tirabrutinib), AVL-292 (spebrutinib); and four reversible BTK inhibitors, CG-806, BMS-935177, BMS-986195, and fenebrutinib. In vitro, TKIs targeting Syk or BTK reduced platelet adhesion to collagen, dense granule secretion, and alpha granule secretion in response to the GPVI agonist cross-linked collagen-related peptide (CRP-XL). Similarly, these TKIs reduced the percentage of activated integrin αIIbβ3 on the platelet surface in response to CRP-XL, as determined by PAC-1 binding. Although all TKIs tested inhibited phospholipase C γ2 (PLCγ2) phosphorylation following GPVI-mediated activation, other downstream signaling events proximal to phosphoinositide 3-kinase (PI3K) and PKC were differentially affected. In addition, reversible BTK inhibitors had less pronounced effects on GPIIb/IIIa-mediated platelet spreading on fibrinogen and differentially altered the organization of PI3K around microtubules during platelets spreading on fibrinogen. Select TKIs also inhibited platelet aggregate formation on collagen under physiological flow conditions. Together, our results suggest that TKIs targeting Syk or BTK inhibit central platelet functional responses but may differentially affect protein activities and organization in critical systems downstream of Syk and BTK in platelets.

Role of oculocerebrorenal syndrome of Lowe (OCRL) protein in megakaryocyte maturation, platelet production and functions: a study in patients with Lowe syndrome

Lowe syndrome (LS) is an oculocerebrorenal syndrome of Lowe (OCRL1) genetic disorder resulting in a defect of the OCRL protein, a phosphatidylinositol-4,5-bisphosphate 5-phosphatase containing various domains including a Rho GTPase-activating protein (RhoGAP) homology domain catalytically inactive. We previously reported surgery-associated bleeding in patients with LS, suggestive of platelet dysfunction, accompanied with a mild thrombocytopenia in several patients. To decipher the role of OCRL in platelet functions and in megakaryocyte (MK) maturation, we conducted a case-control study on 15 patients with LS (NCT01314560). While all had a drastically reduced expression of OCRL, this deficiency did not affect platelet aggregability, but resulted in delayed thrombus formation on collagen under flow conditions, defective platelet spreading on fibrinogen and impaired clot retraction. We evidenced alterations of the myosin light chain phosphorylation (P-MLC), with defective Rac1 activity and, inversely, elevated active RhoA. Altered cytoskeleton dynamics was also observed in cultured patient MKs showing deficient proplatelet extension with increased P-MLC that was confirmed using control MKs transfected with OCRL-specific small interfering(si)RNA (siOCRL). Patients with LS also had an increased proportion of circulating barbell-shaped proplatelets. Our present study establishes that a deficiency of the OCRL protein results in a defective actomyosin cytoskeleton reorganisation in both MKs and platelets, altering both thrombopoiesis and some platelet responses to activation necessary to ensure haemostasis.

Rho GTPase regulation of reactive oxygen species generation and signalling in platelet function and disease

Platelets are master regulators and effectors of haemostasis with increasingly recognized functions as mediators of inflammation and immune responses. The Rho family of GTPase members Rac1, Cdc42 and RhoA are known to be major components of the intracellular signalling network critical to platelet shape change and morphological dynamics, thus playing a major role in platelet spreading, secretion and thrombus formation. Initially linked to the regulation of actomyosin contraction and lamellipodia formation, recent reports have uncovered non-canonical functions of platelet RhoGTPases in the regulation of reactive oxygen species (ROS), where intrinsically generated ROS modulate platelet function and contribute to thrombus formation. Platelet RhoGTPases orchestrate oxidative processes and cytoskeletal rearrangement in an interconnected manner to regulate intracellular signalling networks underlying platelet activity and thrombus formation. Herein we review our current knowledge of the regulation of platelet ROS generation by RhoGTPases and their relationship with platelet cytoskeletal reorganization, activation and function.

Sulfenylome analysis of pathogen-inactivated platelets reveals the presence of cysteine oxidation in integrin signaling pathway and cytoskeleton regulation

Essentials Cysteine oxidation to sulfenic acid plays a key role in redox regulation and signal transduction. Platelet sulfenylome was studied by quantitative proteomics in pathogen inactivated platelets. One hundred and seventy-four sulfenylated proteins were identified in resting platelets. Pathogen inactivation oxidized integrin βIII, which could activate the mitogen-activated protein kinases pathway. ABSTRACT: Background Cysteine-containing protein modifications are involved in numerous biological processes such redox regulation or signal transduction. During the preparation and storage of platelet concentrates, cell functions and protein regulations are impacted. In spite of several proteomic investigations, the platelet sulfenylome, ie, the proteins containing cysteine residues (R-SH) oxidized to sulfenic acid (R-SOH), has not been characterized. Methods A dimedone-based sulfenic acid tagging and enrichment coupled to a mass spectrometry identification workflow was developed to identify and quantify the sulfenic acid-containing proteins in platelet concentrates treated or not with an amotosalen/ultraviolet A (UVA) pathogen inactivation technique. Results One hundred and seventy-four sulfenylated proteins were identified belonging mainly to the integrin signal pathway and cytoskeletal regulation by Rho GTPase. The impact on pathogen inactivated platelet concentrates was weak compared to untreated ones where three sulfenylated proteins (myosin heavy chain 9, integrin βIII, and transgelin 2) were significantly affected by amotosalen/UVA treatment. Of particular interest, the reported oxidation of cysteine residues in integrin βIII is known to activate the receptor αIIbβIII. Following the pathogen inactivation, it might trigger the phosphorylation of p38MAPK and explain the lesions reported in the literature. Moreover, procaspase activating compound-1 (PAC-1) binding assays on platelet activation showed an increased response to adenosine diphosphate exacerbated by the tagging of proteins with dimedone. This result corroborates the hypothesis of an oxidation-triggered activation of αIIbβIII by the pathogen inactivation treatment. Conclusions The present work completes missing information on the platelet proteome and provides new insights on the effect of pathogen inactivation linked to integrin signaling and cytoskeleton regulation.

Phosphoproteomic quantitation and causal analysis reveal pathways in GPVI/ITAM-mediated platelet activation programs

Platelets engage cues of pending vascular injury through coordinated adhesion, secretion, and aggregation responses. These rapid, progressive changes in platelet form and function are orchestrated downstream of specific receptors on the platelet surface and through intracellular signaling mechanisms that remain systematically undefined. This study brings together cell physiological and phosphoproteomics methods to profile signaling mechanisms downstream of the immunotyrosine activation motif (ITAM) platelet collagen receptor GPVI. Peptide tandem mass tag (TMT) labeling, sample multiplexing, synchronous precursor selection (SPS), and triple stage tandem mass spectrometry (MS3) detected >3000 significant (false discovery rate < 0.05) phosphorylation events on >1300 proteins over conditions initiating and progressing GPVI-mediated platelet activation. With literature-guided causal inference tools, >300 site-specific signaling relations were mapped from phosphoproteomics data among key and emerging GPVI effectors (ie, FcRγ, Syk, PLCγ2, PKCδ, DAPP1). Through signaling validation studies and functional screening, other less-characterized targets were also considered within the context of GPVI/ITAM pathways, including Ras/MAPK axis proteins (ie, KSR1, SOS1, STAT1, Hsp27). Highly regulated GPVI/ITAM targets out of context of curated knowledge were also illuminated, including a system of >40 Rab GTPases and associated regulatory proteins, where GPVI-mediated Rab7 S72 phosphorylation and endolysosomal maturation were blocked by TAK1 inhibition. In addition to serving as a model for generating and testing hypotheses from omics datasets, this study puts forth a means to identify hemostatic effectors, biomarkers, and therapeutic targets relevant to thrombosis, vascular inflammation, and other platelet-associated disease states.

The RhoA regulators Myo9b and GEF-H1 are targets of cyclic nucleotide-dependent kinases in platelets

BACKGROUND

Circulating platelets are maintained in an inactive state by the endothelial lining of the vasculature. Endothelium-derived prostacyclin and nitric oxide stimulate cAMP- and cGMP-dependent kinases, PKA and PKG, to inhibit platelets. PKA and PKG effects include the inhibition of the GTPase RhoA, which has been suggested to involve the direct phosphorylation of RhoA on serine 188.

OBJECTIVES

We wanted to confirm RhoA S188 phosphorylation by cyclic nucleotide-dependent kinases and to identify possible alternative mechanisms of RhoA regulation in platelets.

METHODS

Phosphoproteomics data of human platelets were used to identify candidate PKA and PKG substrates. Phosphorylation of individual proteins was studied by Western blotting and Phos-tag gel electrophoresis in human platelets and transfected HEK293T cells. Pull-down assays were performed to analyze protein interaction and function.

RESULTS

Our data indicate that RhoA is not phosphorylated by PKA in platelets. Instead, we provide evidence that cyclic nucleotide effects are mediated through the phosphorylation of the RhoA-specific GTPase-activating protein Myo9b and the guanine nucleotide exchange factor GEF-H1. We identify Myo9b S1354 and guanine nucleotide exchange factor-H1 (GEF-H1) S886 as PKA and PKG phosphorylation sites. Myo9b S1354 phosphorylation enhances its GTPase activating protein function leading to reduced RhoA-GTP levels. GEF-H1 S886 phosphorylation stimulates binding of 14-3-3β and has been shown to inhibit GEF function by facilitating binding of GEF-H1 to microtubules. Microtubule disruption increases RhoA-GTP levels confirming the importance of GEF-H1 in platelets.

CONCLUSION

Phosphorylation of RhoA regulatory proteins Myo9b and GEF-H1, but not RhoA itself, is involved in cyclic nucleotide-mediated control of RhoA in human platelets.

Cholesterol-Rich Microdomains Contribute to PAR1 Signaling in Platelets Despite a Weak Localization of the Receptor in These Microdomains

Platelet protease-activated receptor 1 (PAR1) is a cell surface G-protein-coupled receptor (GPCR) that acts as a thrombin receptor promoting platelet aggregation. Targeting the PAR1 pathway by vorapaxar, a PAR1 antagonist, leads to a reduction in ischemic events in cardiovascular patients with a history of myocardial infarction or with peripheral arterial disease. In platelets, specialized microdomains highly enriched in cholesterol act as modulators of the activity of several GPCRs and play a pivotal role in the signaling pathway. However, their involvement in platelet PAR1 function remains incompletely characterized. In this context, we aimed to investigate whether activation of PAR1 in human platelets requires its localization in the membrane cholesterol-rich microdomains. Using confocal microscopy, biochemical isolation, and proteomics approaches, we found that PAR1 was not localized in cholesterol-rich microdomains in resting platelets, and only a small fraction of the receptor relocated to the microdomains following its activation. Vorapaxar treatment increased the level of PAR1 at the platelet surface, possibly by reducing its endocytosis, while its colocalization with cholesterol-rich microdomains remained weak. Consistent with a cholesterol-dependent activation of Akt and p38 MAP kinase in thrombin receptor-activating peptide (TRAP)-activated platelets, the proteomic data of cholesterol-rich microdomains isolated from TRAP-activated platelets showed the recruitment of proteins contributing to these signaling pathways. In conclusion, contrary to endothelial cells, we found that PAR1 was only weakly present in cholesterol-rich microdomains in human platelets but used these microdomains for efficient activation of downstream signaling pathways following TRAP activation.