Thrombin induces platelet activation in the absence of functional protease activated receptors 1 and 4 and glycoprotein Ib-IX-V

Why platelet mechanotransduction matters for hemostasis and thrombosis

Mechanotransduction is the ability of cells to "feel" or sense their mechanical microenvironment and integrate and convert these physical stimuli into adaptive biochemical cellular responses. This phenomenon is vital for the physiology of numerous nucleated cell types to affect their various cellular processes. As the main drivers of hemostasis and clot retraction, platelets also possess this ability to sense the dynamic mechanical microenvironments of circulation and convert those signals into biological responses integral to clot formation. Like other cell types, platelets leverage their "hands" or receptors/integrins to mechanotransduce important signals in responding to vascular injury to achieve hemostasis. The clinical relevance of cellular mechanics and mechanotransduction is imperative as pathologic alterations or aberrant mechanotransduction in platelets has been shown to lead to bleeding and thrombosis. As such, the aim of this review is to provide an overview of the most recent research related to platelet mechanotransduction, from platelet generation to platelet activation, within the hemodynamic environment and clot contraction at the site of vascular injury, thereby covering the entire "life cycle" of platelets. Additionally, we describe the key mechanoreceptors in platelets and discuss the new biophysical techniques that have enabled the field to understand how platelets sense and respond to their mechanical microenvironment via those receptors. Finally, the clinical significance and importance of continued exploration of platelet mechanotransduction have been discussed as the key to better understanding of both thrombotic and bleeding disorders lies in a more complete mechanistic understanding of platelet function by way of mechanotransduction.

Microparticle Phosphatidylserine Mediates Coagulation: Involvement in Tumor Progression and Metastasis

Tumor progression and cancer metastasis has been linked to the release of microparticles (MPs), which are shed upon cell activation or apoptosis and display parental cell antigens, phospholipids such as phosphatidylserine (PS), and nucleic acids on their external surfaces. In this review, we highlight the biogenesis of MPs as well as the pathophysiological processes of PS externalization and its involvement in coagulation activation. We review the available evidence, suggesting that coagulation factors (mainly tissue factor, thrombin, and fibrin) assist in multiple steps of tumor dissemination, including epithelial-mesenchymal transition, extracellular matrix remodeling, immune escape, and tumor angiogenesis to support the formation of the pre-metastatic niche. Platelets are not just bystander cells in circulation but are functional players in primary tumor growth and metastasis. Tumor-induced platelet aggregation protects circulating tumor cells (CTCs) from the blood flow shear forces and immune cell attack while also promoting the binding of CTCs to endothelial cells and extravasation, which activates tumor invasion and sustains metastasis. Finally, in terms of therapy, lactadherin can inhibit coagulation by competing effectively with coagulation factors for PS binding sites and may similarly delay tumor progression. Furthermore, we also investigate the therapeutic potential of coagulation factor inhibitors within the context of cancer treatment. The development of multiple therapies targeting platelet activation and platelet-tumor cell interactions may not only reduce the lethal consequences of thrombosis but also impede tumor growth and spread.

The GPIb-IX complex on platelets: insight into its novel physiological functions affecting immune surveillance, hepatic thrombopoietin generation, platelet clearance and its relevance for cancer development and metastasis

The glycoprotein (GP) Ib-IX complex is a platelet receptor that mediates the initial interaction with subendothelial von Willebrand factor (VWF) causing platelet arrest at sites of vascular injury even under conditions of high shear. GPIb-IX dysfunction or deficiency is the reason for the rare but severe Bernard-Soulier syndrome (BSS), a congenital bleeding disorder. Although knowledge on GPIb-IX structure, its basic functions, ligands, and intracellular signaling cascades have been well established, several advances in GPIb-IX biology have been made in the recent years. Thus, two mechanosensitive domains and a trigger sequence in GPIb were characterized and its role as a thrombin receptor was deciphered. Furthermore, it became clear that GPIb-IX is involved in the regulation of platelet production, clearance and thrombopoietin secretion. GPIb is deemed to contribute to liver cancer development and metastasis. This review recapitulates these novel findings highlighting GPIb-IX in its multiple functions as a key for immune regulation, host defense, and liver cancer development.

Fibrillar amyloid peptides promote platelet aggregation through the coordinated action of ITAM- and ROS-dependent pathways

BACKGROUND

Amyloid peptides Aβ40 and Aβ42, whose deposition in brain correlates with Alzheimer disease, are also present in platelets and have prothrombotic activities.

OBJECTIVE

In this study, we analyze the ability of Aβ peptides to form fibrils and to induce platelet activation and aggregation.

METHODS

Aβ40, Aβ42, and their scrambled peptides were diluted in phosphate buffered saline and fibrillogenesis was investigated by ThioflavinT and Congo Red. Aggregation, protein phosphorylation, and reactive oxygen species (ROS) production were analyzed.

RESULTS

Aβ40 and Aβ42, but not scrambled peptides, were able to form fibrils when diluted in phosphate buffered saline. Fibrillogenesis of Aβ42 was very rapid, whereas fibril formation by Aβ40 was completed only after 48 hours of incubation. Fibrillar Aβ40 and Aβ42 promoted dose-dependent aggregation of washed platelets in the presence of extracellular CaCl2 . Cleavage of GPIbα by mocarhagin or blockade of the ITAM-containing FcγRIIA prevented platelet aggregation induced by fibrillary Aβ40 and Aβ42. Fibrillar Aβ peptides stimulated the phosphorylation of FcγRIIA, resulting in the downstream stimulation of PLC, protein kinase C, and phosphoinositide 3-kinases, whose activity was necessary for full aggregation of platelets. Fibrillar Aβ peptides also induced ROS generation, and NOX inhibitors, as well as ROS scavengers, prevented platelet aggregation. However, Aβ peptide-induced ROS production did not require binding to GPIbα or activation of FcγRIIA, but was initiated by CD36, which provided an important contribution to full platelet aggregation.

CONCLUSION

These results suggest that fibrillar amyloid Aβ40 and Aβ42 induce platelet aggregation through the recruitment of GPIb-IX-V and CD36, which requires the convergence of ITAM- and ROS-dependent pathways.

GPR56/ADGRG1 is a platelet collagen-responsive GPCR and hemostatic sensor of shear force

We identified the known collagen receptor GPR56/ADGRG1 on platelets. GPR56 is an adhesion G protein-coupled receptor that becomes activated following forced dissociation of its N-terminal fragment and C-terminal fragment or seven-transmembrane spanning domain (7TM). Fragment dissociation reveals the cryptic stalk of the 7TM, which acts as a tethered peptide agonist, and for GPR56, this activates platelet G13 signaling. GPR56 pharmacological probes activated platelets to undergo shape change and aggregation, which are critical for the formation of hemostatic plugs. Gpr56^−/− mice exhibit prolonged bleeding, defective platelet plug formation in vessel injury assays, and delayed thrombotic vessel occlusion. Shear-force dependency of platelet adhesion to immobilized collagen was found to be GPR56 dependent.

Circulating platelets roll along exposed collagen at vessel injury sites and respond with filipodia protrusion, shape change, and surface area expansion to facilitate platelet adhesion and plug formation. Various glycoproteins were considered to be both collagen responders and mediators of platelet adhesion, yet the signaling kinetics emanating from these receptors do not fully account for the rapid platelet cytoskeletal changes that occur in blood flow. We found the free N-terminal fragment of the adhesion G protein-coupled receptor (GPCR) GPR56 in human plasma and report that GPR56 is the platelet receptor that transduces signals from collagen and blood flow-induced shear force to activate G protein 13 signaling for platelet shape change. Gpr56^−/− mice have prolonged bleeding, defective platelet plug formation, and delayed thrombotic occlusion. Human and mouse blood perfusion studies demonstrated GPR56 and shear-force dependence of platelet adhesion to immobilized collagen. Our work places GPR56 as an initial collagen responder and shear-force transducer that is essential for platelet shape change during hemostasis.

Pulmonary Artery Thrombosis: A Diagnosis That Strives for Its Independence

According to a widespread theory, thrombotic masses are not formed in the pulmonary artery (PA) but result from migration of blood clots from the venous system. This concept has prevailed in clinical practice for more than a century. However, a new technologic era has brought forth more diagnostic possibilities, and it has been shown that thrombotic masses in the PA could, in many cases, be found without any obvious source of emboli. Chronic obstructive pulmonary disease, asthma, sickle cell anemia, emergency and elective surgery, viral pneumonia, and other conditions could be complicated by PA thrombosis development without concomitant deep vein thrombosis (DVT). Different pathologies have different causes for local PA thrombotic process. As evidenced by experimental results and clinical observations, endothelial and platelet activation are the crucial mechanisms of this process. Endothelial dysfunction can impair antithrombotic function of the arterial wall through downregulation of endothelial nitric oxide synthase (eNOS) or via stimulation of adhesion receptor expression. Hypoxia, proinflammatory cytokines, or genetic mutations may underlie the procoagulant phenotype of the PA endothelium. Both endotheliocytes and platelets could be activated by protease mediated receptor (PAR)- and receptors for advanced glycation end (RAGE)-dependent mechanisms. Hypoxia, in particular induced by high altitudes, could play a role in thrombotic complications as a trigger of platelet activity. In this review, we discuss potential mechanisms of PA thrombosis in situ.

Changes in platelet Bax levels contribute to impaired platelet response to thrombin after cardiopulmonary bypass: prospective observational clinical and laboratory investigations

Background

Anucleate platelets can undergo apoptosis in response to various stimuli, as do nucleated cells. Cardiopulmonary bypass (CPB) causes platelet dysfunction and can also activate platelet apoptotic pathways. We therefore evaluated time-dependent changes in blood platelet Bax (a pro-apoptotic molecule) levels and platelet dysfunction after cardiac surgery.

Methods

We assessed blood samples obtained from subjects having on-pump or off-pump coronary artery bypass graft surgery ( n =20 each). We also evaluated the in vitro effects of platelet Bax increase in eight healthy volunteers.

Results

Thrombin-induced platelet calcium mobilisation and platelet-surface glycoprotein Ib (GPIb) expression were lowest at weaning from CPB and did not recover on postoperative day one. On-pump surgery increased platelet expression of Bax, especially the oligomerised form, along with translocation of Bax from the cytosol to mitochondria and platelet-surface tumour necrosis factor-alpha (TNF-α)-converting enzyme (TACE) expression. In contrast, mitochondrial cytochrome c expression was reduced. While similar in direction, the magnitude of the observed changes was smaller in patients having off-pump surgery. In vitro , a cell-permeable Bax peptide increased platelet Bax expression to the same extent seen during bypass and produced similar platelet changes. These apoptotic-like changes were largely reversed by Bcl-xL pre-administration, and were completely reversed by combined application of inhibitors that stabilise outer mitochondrial membrane permeability and TACE.

Conclusions

CPB increases platelet Bax expression, which contributes to reduced platelet-surface GPIb expression and thrombin-induced platelet calcium changes. These changes in platelet apoptotic signalling might contribute to platelet dysfunction after CPB.

Clinical trial registration

UMIN Clinical Trials Registry (number UMIN000006033).

Thrombin-unique coagulation system protein with multifaceted impacts on cancer and metastasis

The association between blood coagulation and cancer development is well recognized. Thrombin, the pleiotropic enzyme best known for its contribution to fibrin formation and platelet aggregation during vascular hemostasis, may also trigger cellular events through protease-activated receptors, PAR-1 and PAR-4, leading to cancer progression. Our pioneering findings provided evidence that thrombin contributes to cancer metastasis by increasing adhesive potential of malignant cells. However, there is evidence that thrombin regulates every step of cancer dissemination: (1) cancer cell invasion, detachment from primary tumor, migration; (2) entering the blood vessel; (3) surviving in vasculature; (4) extravasation; (5) implantation in host organs. Recent studies have provided new molecular data about thrombin generation in cancer patients and the mechanisms by which thrombin contributes to transendothelial migration, platelet/tumor cell interactions, angiogenesis, and other processes. Though a great deal is known regarding the role of thrombin in cancer dissemination, there are new data for multiple thrombin-mediated events that justify devoting focus to this topic with a comprehensive approach.

Thrombin-induced reactive oxygen species generation in platelets: A novel role for protease-activated receptor 4 and GPIbα

Background

Platelets are essential for maintaining haemostasis and play a key role in the pathogenesis of cardiovascular disease. Upon ligation of platelet receptors through subendothelial matrix proteins, intracellular reactive oxygen species (ROS) are generated, further amplifying the platelet activation response. Thrombin, a potent platelet activator, can signal through GPIbα and protease-activated receptor (PAR) 1 and PAR4 on human platelets, and recently has been implicated in the generation of ROS. While ROS are known to have key roles in intra-platelet signalling and subsequent platelet activation, the precise receptors and signalling pathways involved in thrombin-induced ROS generation have yet to be fully elucidated.

Objective

To investigate the relative contribution of platelet GPIbα and PARs to thrombin-induced reactive oxygen species (ROS) generation.

Methods and results

Highly specific antagonists targeting PAR1 and PAR4, and the GPIbα-cleaving enzyme, Naja kaouthia (Nk) protease, were used in quantitative flow cytometry assays of thrombin-induced ROS production. Antagonists of PAR4 but not PAR1, inhibited thrombin-derived ROS generation. Removal of the GPIbα ligand binding region attenuated PAR4-induced and completely inhibited thrombin-induced ROS formation. Similarly, PAR4 deficiency in mice abolished thrombin-induced ROS generation. Additionally, GPIbα and PAR4-dependent ROS formation were shown to be mediated through focal adhesion kinase (FAK) and NADPH oxidase 1 (NOX1) proteins.

Conclusions

Both GPIbα and PAR4 are required for thrombin-induced ROS formation, suggesting a novel functional cooperation between GPIbα and PAR4. Our study identifies a novel role for PAR4 in mediating thrombin-induced ROS production that was not shared by PAR1. This suggests an independent signalling pathway in platelet activation that may be targeted therapeutically.

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PAR4 plays an important role in platelet-derived ROS generation.

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Thrombin-induced ROS generation in platelets require both GPIbα and PAR4.

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Potential functional association between GPIbα and PAR4 receptors and mouse and human platelets.

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GPIbα and PAR4-dependent ROS formation is mediated through FAK and NOX1 proteins.

Thrombin-targeted liposomes establish a sustained localized anticlotting barrier against acute thrombosis

The goal of the present work was to design and test an acute-use nanoparticle-based antithrombotic agent that exhibits sustained local inhibition of thrombin without requiring a systemic anticoagulant effect to function against acute arterial thrombosis. To demonstrate proof of concept, we functionalized the surface of liposomes with multiple copies of the direct thrombin inhibitor, d-phenylalanyl-l-prolyl-l-arginyl-chloromethyl ketone (PPACK), which exhibits high affinity for thrombin as a free agent but manifests too rapid clearance in vivo to be effective alone. The PPACK-liposomes were formulated as single unilamellar vesicles, with a diameter of 170.78 ± 10.59 nm and a near neutral charge. In vitro models confirmed the inhibitory activity of PPACK-liposomes, demonstrating a KI' of 172.6 nM. In experimental clots in vitro, treatment of formed clots completely abrogated any further clotting upon exposure to human plasma. The liposomes were evaluated in vivo in a model of photochemical-induced carotid artery injury, resulting in significantly prolonged arterial occlusion time over that of controls (69.06 ± 5.65 min for saline treatment, N = 6, 71.33 ± 9.46 min for free PPACK treated; N = 4, 85.75 ± 18.24 min for precursor liposomes; N = 4, 139.75 ± 20.46 min for PPACK-liposomes; P = 0.0049, N = 6). Systemic anticoagulant profiles revealed a rapid return to control levels within 50 min, while still maintaining antithrombin activity at the injury site. The establishment of a potent and long-acting anticoagulant surface over a newly forming clot with the use of thrombin targeted nanoparticles that do not require systemic anticoagulation to be effective offers an alternative site-targeted approach to the management of acute thrombosis.

Trypsin causes platelet activation independently of known protease-activated receptors

To identify a physiological agonist of PAR3, we used PAR4 null murine platelets, which were known to express only PAR3. In this study, we tested several proteases and found that trypsin, but not heat-inactivated trypsin, activated PAR4 null murine platelets. Even at high concentrations, trypsin caused shape change without increasing intracellular calcium levels in PAR4 null murine platelets. Consistent with this result, the Gq inhibitor YM-254890 had no effect on trypsin-induced shape change. However, trypsin-induced platelet shape change was abolished by either p160ROCK inhibitor, Y27632 or H1152. Furthermore, trypsin caused phosphorylation of myosin light chain (Thr18), but not Akt or Erk. Surprisingly, trypsin caused a similar shape change in PAR4-desensitised PAR3 null murine platelets as in PAR4null murine platelets, indicating that trypsin did not activate PAR3 to cause shape change. More interestingly, the Src family kinase (SFK) inhibitor PP2 abolished trypsin-induced, but not AYPGKF-induced, shape change. Hence, trypsin activated a novel signalling pathway through RhoA/p160ROCK and was regulated by SFKs. In conclusion, our study demonstrates a novel protease signalling pathway in platelets that is independent of PARs. This protease-induced novel signalling pathway regulates platelet shape change through SFKs and p160ROCK.

PAR-1-dependent and PAR-independent pro-inflammatory signaling in human lung fibroblasts exposed to thrombin

Proteinase-activated receptors (PARs) are crucial in orchestrating cellular responses to coagulation proteinases, such as thrombin and FXa. Four PARs have been characterized and have been shown to be differentially expressed in mice and humans and between tissues. We have previously shown that in murine lung fibroblasts, PAR-1 is solely responsible for all cellular responses to thrombin and FXa. In contrast, we report here that in primary human lung fibroblasts (pHLFs), known PARs fail to account for all of the cellular responses to thrombin, in particular in the presence of high, but physiologically achievable concentrations of thrombin. We report that pHLFs secrete CCL2 in a PAR-1-dependent manner at low thrombin concentration (∼0.3 nM). At or above 10 nM thrombin, pharmacological antagonism (RWJ-58259) fails to block thrombin-induced CCL2 release; whereas PAR-1 cleavage-blocking monoclonal antibodies (ATAP2 and WEDE15) only partially inhibit thrombin-induced CCL2 secretion. In addition, activation of PAR-3, PAR-4, and transactivation of either PAR-2 or EGFR were ruled out as being responsible for thrombin-mediated CCL2 secretion at high yet standard concentrations of the proteinase. We further provide evidence that PAR-1-dependent and PAR-independent signaling involves the rapid phosphorylation of ERK, which in turn is absolutely required for thrombin-induced CCL2 secretion at both low and standard concentration of the proteinase. Our findings suggest the existence of a PAR-independent signaling mechanism in human lung fibroblasts and have important implications for the design of therapeutic strategies aimed at blocking pro-inflammatory signaling responses associated with excessive thrombin generation.

Atopaxar: a review of its mechanism of action and role in patients with coronary artery disease

Platelet activation and aggregation is a complex and key process in thrombus formation after the rupture of an atherosclerotic plaque, which can lead to an acute coronary syndrome. Aspirin, an irreversible inhibitor of thromboxane A2 synthesis, in combination with an inhibitor of P2Y12 ADP platelet receptors (clopidogrel, prasugrel or ticagrelor), represents the current standard of care of antiplatelet therapy for patients with acute coronary syndrome and in those patients undergoing percutaneous coronary intervention. Despite the benefit of these agents, the risk of thrombotic events and bleeding complications may still occur while on such antiplatelet treatment regimens, thus representing an important limitation. Thrombin is one of the most important platelet activators. The inhibition of thrombin-mediated platelet activation by means of protease-activated receptor-1 inhibitors represents an attractive therapeutic option for patients with atherothrombotic disease processes. This article provides an overview on atopaxar (E5555), an orally active protease-activated receptor-1 antagonist that has recently completed Phase II clinical investigation.

Thrombin receptor levels in platelet concentrates during storage and their impact on platelet functionality

BACKGROUND

Quality control of platelet (PLT) concentrates is challenging, due to PLT lesions, which are difficult to detect with routine methods. The search for reliable PLT lesion biomarkers is focused on the role of PLTs in primary hemostasis. PLT transfusions also have a significant impact on secondary hemostasis. In this phase, responsiveness of PLTs to small amounts of thrombin is crucial. PAR1 and PAR4 are protease-activated receptors and are responsible for thrombin reactivity of human PLTs. This study should elucidate if levels of those two receptors are changing in PLT concentrates during storage and if those changes have an impact on PLT aggregation and support of thrombin generation.

STUDY DESIGN AND METHODS

PLT concentrates from buffy coat preparations were stored in SSP+ solution for 9 days at 22±2°C on a horizontal flatbed agitator, and samples were taken daily for analysis. PAR1 and PAR4 levels were evaluated using Western blot analysis. PLT aggregation was measured using Born aggregometry and specific PAR1 or PAR4 agonists. Thrombin generation was measured using calibrated automated thrombography.

RESULTS

Levels of both receptors (PAR1 and PAR4) started to decrease after 5 days of storage. PAR1-mediated PLT aggregation remained constant, whereas PAR4-mediated PLT aggregation decreased with storage time. Rate of thrombin generation was accelerated after 5 days of storage.

CONCLUSION

Decreasing levels of PARs in PLT concentrates after 5 days of storage influenced PAR4-mediated, but not PAR1-mediated, aggregation. Thrombin generation with senescent PLTs was increased, which may be attributed to other mechanisms promoting increased phosphatidylserine exposure.

Thrombin as a multi-functional enzyme. Focus on in vitro and in vivo effects

Thrombin is the central protease in the coagulation cascade and one of the most extensively studied of all enzymes. In addition to its recognised role in the coagulation cascade and haemostasis, thrombin is known to have multiple pleiotropic effects, which mostly have been shown only in in vitro studies: it plays a role in inflammation and cellular proliferation and displays a mitogen activity on smooth muscle cells and endothelial cells, predominantly by activation of angiogenesis. In vivo , thrombin effects were examined in animal models of intravenous or intraarterial thrombin infusion. An extensive literature search regarding in vivo data showed that i) thrombin administered as a bolus causes microembolism, ii) thrombin infused slowly at steady-state conditions (up to 1.6 U/kg/min) leads to bleeds but not to intravascular clotting, iii) large quantity of thrombin infused at low rates (0.05 U/kg/min) does not have any measurable effect, and iv) thrombin increases vascular permeability leading to tissue damage. Although several decades of research on thrombin functions have provided a framework for understanding the biology of thrombin, animal and human studies with use of newer laboratory techniques are still needed to confirm the pleiotropic thrombin functions shown in in vitro studies.

Thrombin and vascular inflammation

Vascular endothelium is a key regulator of homeostasis. In physiological conditions it mediates vascular dilatation, prevents platelet adhesion, and inhibits thrombin generation. However, endothelial dysfunction caused by physical injury of the vascular wall, for example during balloon angioplasty, acute or chronic inflammation, such as in atherothrombosis, creates a proinflammatory environment which supports leukocyte transmigration toward inflammatory sites. At the same time, the dysfunction promotes thrombin generation, fibrin deposition, and coagulation. The serine protease thrombin plays a pivotal role in the coagulation cascade. However, thrombin is not only the key effector of coagulation cascade; it also plays a significant role in inflammatory diseases. It shows an array of effects on endothelial cells, vascular smooth muscle cells, monocytes, and platelets, all of which participate in the vascular pathophysiology such as atherothrombosis. Therefore, thrombin can be considered as an important modulatory molecule of vascular homeostasis. This review summarizes the existing evidence on the role of thrombin in vascular inflammation.