Diverging signaling events control the pathway of GPVI down-regulation in vivo

Upregulation of olfactory receptors and neuronal-associated genes highlights complex immune and neuronal dysregulation in Long COVID patients

A substantial portion of patients infected with SARS-CoV-2 experience prolonged complications, known as Long COVID (LC). A subset of these patients exhibits the most debilitating symptoms, similar to those defined in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS). We performed bulk RNA sequencing (RNAseq) on the whole blood of LC with ME/CFS, at least 12 months post-onset of the acute disease, and compared them with controls. We found that LC patients had a distinct transcriptional profile compared to controls. Key findings include the upregulation of genes involved in immune dysregulation and neuronal development, such as Fezf2, BRINP2, HOXC12, MEIS2, ZFHX3, and RELN. These genes are linked to neuroinflammatory responses, cognitive impairments, and hematopoietic disturbances, suggesting ongoing neurological and immune disturbances in LC patients. RELN, encoding the Reelin protein, was notably elevated in LC patients, potentially serving as a biomarker for LC pathogenesis due to its role in inflammation and neuronal function. Immune cell analysis showed altered profiles in LC patients, with increased activated memory CD4 + T cells and neutrophils, and decreased regulatory T cells and NK cells, reflecting immune dysregulation. Changes in cytokine and chemokine expression further underscore the chronic inflammatory state in LC patients. Notably, a unique upregulation of olfactory receptors (ORs) suggest alternative roles for ORs in non-olfactory tissues. Pathway analysis revealed upregulation in ribosomal RNA processing, amino acid metabolism, protein synthesis, cell proliferation, DNA repair, and mitochondrial pathways, indicating heightened metabolic and immune demands. Conversely, downregulated pathways, such as VEGF signaling and TP53 activity, point to impaired tissue repair and cellular stress responses. Overall, our study underscores the complex interplay between immune and neuronal dysfunction in LC patients, providing insights into potential diagnostic biomarkers and therapeutic targets. Future research is needed to fully understand the roles and interactions of these genes in LC pathophysiology.

Platelet GPVI (Glycoprotein VI) and Thrombotic Complications in the Venous System

The immunoglobulin receptor GPVI (glycoprotein VI) is selectively expressed on megakaryocytes and platelets and is currently recognized as a receptor for not only collagen but also a variety of plasma and vascular proteins, including fibrin, fibrinogen, laminin, fibronectin, and galectin-3. Deficiency of GPVI is protective in mouse models of experimental thrombosis, pulmonary thromboembolism as well as in thromboinflammation, suggesting a role of GPVI in arterial and venous thrombus formation. In humans, platelet GPVI deficiency is associated with a mild bleeding phenotype, whereas a common variant rs1613662 in the GP6 gene is considered a risk factor for venous thromboembolism. However, preclinical studies on the inhibition of GPVI-ligand interactions are focused on arterial thrombotic complications. In this review we discuss the emerging evidence for GPVI in venous thrombus formation and leukocyte-dependent thromboinflammation, extending to venous thromboembolism, pulmonary thromboembolism, and cancer metastasis. We also recapitulate indications for circulating soluble GPVI as a biomarker of thrombosis-related complications. Collectively, we conclude that the current evidence suggests that platelet GPVI is also a suitable cotarget in the prevention of venous thrombosis due to its role in thrombus consolidation and platelet-leukocyte complex formation.

Hemostasis vs. homeostasis: Platelets are essential for preserving vascular barrier function in the absence of injury or inflammation

Platelets are best known for their vasoprotective responses to injury and inflammation. Here, we have asked whether they also support vascular integrity when neither injury nor inflammation is present. Changes in vascular barrier function in dermal and meningeal vessels were measured in real time in mouse models using the differential extravasation of fluorescent tracers as a biomarker. Severe thrombocytopenia produced by two distinct methods caused increased extravasation of 40-kDa dextran from capillaries and postcapillary venules but had no effect on extravasation of 70-kDa dextran or albumin. This reduction in barrier function required more than 4 h to emerge after thrombocytopenia was established, reverting to normal as the platelet count recovered. Barrier dysfunction was also observed in mice that lacked platelet-dense granules, dense granule secretion machinery, glycoprotein (GP) VI, or the GPVI signaling effector phospholipase C (PLC) γ2. It did not occur in mice lacking α-granules, C type lectin receptor-2 (CLEC-2), or protease activated receptor 4 (PAR4). Notably, although both meningeal and dermal vessels were affected, intracerebral vessels, which are known for their tighter junctions between endothelial cells, were not. Collectively, these observations 1) highlight a role for platelets in maintaining vascular homeostasis in the absence of injury or inflammation, 2) provide a sensitive biomarker for detecting changes in platelet-dependent barrier function, 3) identify which platelet processes are required, and 4) suggest that the absence of competent platelets causes changes in the vessel wall itself, accounting for the time required for dysfunction to emerge.

Absence of bleeding upon dual antiplatelet therapy in a patient with a immune GPVI deficiency

Acquired deficiencies in platelet glycoprotein VI are rare and have not been found associated with other defects. Here we report the case of a 64-year old male patient presenting an immune GPVI deficiency associated to a mutation in the alpha-actinin gene and who has been treated with dual anti platelet therapy without bleeding.Introduction: Glycoprotein (GP) VI, a pluripotent receptor interacting with collagen and fibrin(ogen) is responsible for thrombus formation, growth and stability (1-4). It is co-expressed with the Fc receptor γ (FcRγ) chain (5). GPVI is not critical for haemostasis since subjects with a GPVI deficiency usually present low or even no bleeding tendency (6, 7). Acquired GPVI deficiency due to antibody-induced GPVI depletion is the most frequent finding. At least 10 patients have been described with an acquired GPVI deficiency, most often associated to immune thrombocytopenia, moderate bleeding and impaired collagen-induced platelet aggregation (7). Several mechanisms leading to the GPVI deficiency are proposed including antibody-triggered GPVI internalization and/or shedding of the extracellular domain (8, 9). We report the case of a patient presenting an acquired GPVI deficiency different from those previously described: (i) he is male whereas all previous cases were female, (ii) he is heterozygous for a mutation in α (alpha)-actinin-1 gene and (iii) he was treated with dual antiplatelet therapy with no haemorrhagic manifestation.

Clinical significance of altered collagen-receptor functioning in platelets with emphasis on glycoprotein VI

Much interest surrounds the receptors α2β1 and glycoprotein VI (GPVI) whose synchronized action mediates the attachment and activation of platelets on collagen, essential for preventing blood loss but also the most thrombogenic component of the vessel wall. Subject to density variations on platelets through natural polymorphisms, the absence of α2β1 or GPVI uniquely leads to a substantial block of hemostasis without causing major bleeding. Specific to the megakaryocyte lineage, GPVI and its signaling pathways are most promising targets for anti-thrombotic therapy. This review looks at the clinical consequences of the loss of collagen receptor function with emphasis on both the inherited and acquired loss of GPVI with brief mention of mouse models when necessary. A detailed survey of rare case reports of patients with inherited disease-causing variants of the GP6 gene is followed by an assessment of the causes and clinical consequences of acquired GPVI deficiency, a more frequent finding most often due to antibody-induced platelet GPVI shedding. Release of soluble GPVI is brought about by platelet metalloproteinases; a process induced by ligand or antibody binding to GPVI or even high shear forces. Also included is an assessment of the clinical importance of GPVI-mediated platelet interactions with fibrin and of the promise shown by the pharmacological inhibition of GPVI in a cardiovascular context. The role for GPVI in platelet function in inflammation and in the evolution and treatment of major illnesses such as rheumatoid arthritis, cancer and sepsis is also discussed.

Dynamin 2 is required for GPVI signaling and platelet hemostatic function in mice

Receptor-mediated endocytosis, which contributes to a wide range of cellular functions, including receptor signaling, cell adhesion, and migration, requires endocytic vesicle release by the large GTPase dynamin 2. Here, the role of dynamin 2 was investigated in platelet hemostatic function using both pharmacological and genetic approaches. Dnm2^fl/fl Pf4-Cre (Dnm2^Plt−/−) mice specifically lacking dynamin 2 within the platelet lineage developed severe thrombocytopenia and bleeding diathesis and Dnm2^Plt−/− platelets adhered poorly to collagen under arterial shear rates. Signaling via the collagen receptor GPVI was impaired in platelets treated with the dynamin GTPase inhibitor dynasore, as evidenced by poor protein tyrosine phosphorylation, including that of the proximal tyrosine kinase Lyn on its activating tyrosine 396 residue. Platelet stimulation via GPVI resulted in a slight decrease in GPVI, which was maintained by dynasore treatment. Dynasore-treated platelets had attenuated function when stimulated via GPVI, as evidenced by reduced GPIbα downregulation, α-granule release, integrin αIIbβ3 activation, and spreading onto immobilized fibrinogen. By contrast, responses to the G-protein coupled receptor agonist thrombin were minimally affected by dynasore treatment. GPVI expression was severely reduced in Dnm2^Plt−/− platelets, which were dysfunctional in response to stimulation via GPVI, and to a lesser extent to thrombin. Dnm2^Plt−/− platelets lacked fibrinogen in their α-granules, but retained von Willebrand factor. Taken together, the data show that dynamin 2 plays a proximal role in signaling via the collagen receptor GPVI and is required for fibrinogen uptake and normal platelet hemostatic function.

Soluble GPVI is elevated in injured patients: shedding is mediated by fibrin activation of GPVI

Soluble glycoprotein VI (sGPVI) is shed from the platelet surface and is a marker of platelet activation in thrombotic conditions. We assessed sGPVI levels together with patient and clinical parameters in acute and chronic inflammatory conditions, including patients with thermal injury and inflammatory bowel disease and patients admitted to the intensive care unit (ICU) for elective cardiac surgery, trauma, acute brain injury, or prolonged ventilation. Plasma sGPVI was measured by enzyme-linked immunosorbent assay and was elevated on day 14 after thermal injury, and was higher in patients who developed sepsis. sGPVI levels were associated with sepsis, and the value for predicting sepsis was increased in combination with platelet count and Abbreviated Burn Severity Index. sGPVI levels positively correlated with levels of D-dimer (a fibrin degradation product) in ICU patients and patients with thermal injury. sGPVI levels in ICU patients at admission were significantly associated with 28- and 90-day mortality independent of platelet count. sGPVI levels in patients with thermal injury were associated with 28-day mortality at days 1, 14, and 21 when adjusting for platelet count. In both cohorts, sGPVI associations with mortality were stronger than D-dimer levels. Mechanistically, release of GPVI was triggered by exposure of platelets to polymerized fibrin, but not by engagement of G protein-coupled receptors by thrombin, adenosine 5'-diphosphate, or thromboxane mimetics. Enhanced fibrin production in these patients may therefore contribute to the observed elevated sGPVI levels. sGPVI is an important platelet-specific marker for platelet activation that predicts sepsis progression and mortality in injured patients.

A unique phenotype of acquired Glanzmann thrombasthenia due to non-function-blocking anti-αIIbβ3 autoantibodies

Essentials Acquired Glanzmann thrombasthenia (aGT) is generally caused by function-blocking antibodies (Abs). We demonstrated a unique aGT case due to marked reduction of αIIbβ3 with anti-αIIbβ3 Abs. The anti-αIIbβ3 Abs of the patient did not inhibit platelet function but reduced surface αIIbβ3. Internalization of αIIbβ3 induced by the Abs binding may be responsible for the phenotype. SUMMARY: Background Acquired Glanzmann thrombasthenia (aGT) is a bleeding disorder generally caused by function-blocking anti-αIIbβ3 autoantibodies. Aim We characterize an unusual case of aGT caused by marked reduction of surface αIIbβ3 with non-function-blocking anti-αIIbβ3 antibodies (Abs). Methods A 72-year-old male suffering from immune thrombocytopenia since his 50s showed exacerbation of bleeding symptom despite mild thrombocytopenia. Platelet aggregation was absent with all agonists but ristocetin. Analysis of αIIbβ3 expression and genetic analysis were performed. We also analyzed effects of anti-αIIbβ3 Abs of the patient on platelet function and αIIbβ3 expression. Results Surface αIIbβ3 expression was markedly reduced to around 5% of normal, whereas his platelets contained αIIbβ3 to the amount of 40-50% of normal. A substantial amount of fibrinogen was also detected in his platelets. There were no abnormalities in ITGA2B and ITGB3 cDNA. These results indicated that reduced surface αIIbβ3 expression caused a GT phenotype, and active internalization of αIIbβ3 was suggested. Anti-αIIbβ3 IgG Abs were detected in platelet eluate and plasma. These Abs did not inhibit PAC-1 binding, indicating that the Abs were non-function-blocking. Surface αIIbβ3 expression of a megakaryocytic cell line and cultured megakaryocytes tended to be impaired by incubation with the patient's Abs. After 2 years of aGT diagnosis, his bleeding symptom improved and surface αIIbβ3 expression was recovered to 20% of normal with reduction of anti-αIIbβ3 Abs. Conclusion We demonstrated a unique aGT phenotype due to marked reduction of surface αIIbβ3. Internalization induced by anti-αIIbβ3 Abs may be responsible in part for the phenotype.

Proteolytic processing of platelet receptors

Platelets have a major role in hemostasis and an emerging role in biological processes including inflammation and immunity. Many of these processes require platelet adhesion and localization at sites of tissue damage or infection and regulated platelet activation, mediated by platelet adheso-signalling receptors, glycoprotein (GP) Ib-IX-V and GPVI. Work from a number of laboratories has demonstrated that levels of these receptors are closely regulated by metalloproteinases of the A Disintegrin And Metalloproteinase (ADAM) family, primarily ADAM17 and ADAM10. It is becoming increasingly evident that platelets have important roles in innate immunity, inflammation, and in combating infection that extends beyond processes of hemostasis. This overview will examine the molecular events that regulate levels of platelet receptors and then assess ramifications for these events in settings where hemostasis, inflammation, and infection processes are triggered.

Targeting platelet receptors in thrombotic and thrombo-inflammatory disorders

Platelet activation at sites of vascular injury is critical for the formation of a hemostatic plug which limits excessive blood loss, but also represents a major pathomechanism of ischemic cardio- and cerebrovascular diseases. Although currently available antiplatelet therapies have proved beneficial in preventing the recurrence of vascular events, their adverse effects on primary hemostasis emphasize the necessity to identify and characterize novel pharmacological targets for platelet inhibition. Increasing experimental evidence has suggested that several major platelet surface receptors which regulate initial steps of platelet adhesion and activation may become promising new targets for anti-platelet drugs due to their involvement in thrombotic and thrombo-inflammatory signaling cascades.

This review summarizes recent developments in understanding the function of glycoprotein (GP)Ib, GPVI and the C-type lectin-like receptor 2 (CLEC-2) in hemostasis, arterial thrombosis and thrombo-inflammation and will discuss the suitability of the receptors as novel targets to treat these diseases in humans.

Comparison of two murine models of thrombosis induced by atherosclerotic plaque injury

Arterial thrombosis occurs at sites of erosion or rupture of atherosclerotic vascular lesions. To better study the pathophysiology of this complex phenomenon, there is a need for animal models of localised thrombosis at sites of atherosclerotic lesions with closer resemblance to the human pathology as compared to commonly used thrombosis models in healthy vessels. In the present study, we describe and compare a new model of thrombosis induced by atherosclerotic plaque rupture in the carotid artery from ApoE-/- mice using a suture needle to a milder model of ultrasound-induced plaque injury. Needle injury induces atherosclerotic plaque rupture with exposure of plaque material and formation of a thrombus that is larger, nearly occlusive and more stable as compared to that formed by application of ultrasounds. These two models have common features such as the concomitant involvement of platelet activation, thrombin generation and fibrin formation, which translates into sensitivity toward both antiplatelet drugs and anticoagulants. On the other hand, they display differences with respect to the role of the platelet collagen receptor GPVI, the plaque rupture model being less sensitive to its inhibition as compared to the ultrasound-induced injury, which may be related to the amount of thrombin generated. These models represent an improvement as compared to models in healthy vessels and may help identify specific plaque triggers of thrombosis. They should therefore be useful to evaluate new antithrombotic targets.

Focusing on plasma glycoprotein VI

New methods for analysing both platelet and plasma forms of the platelet-specific collagen receptor, glycoprotein VI (GPVI) in experimental models or human clinical samples, and the development of the first therapeutic compounds based on dimeric soluble GPVI-Fc or anti-GPVI antibody-based constructs, coincide with increased understanding of the potential pathophysiological role of GPVI ligand binding and shedding. Platelet GPVI not only mediates platelet activation at the site of vascular injury where collagen is exposed, but is also implicated in the pathogenesis of other diseases, such as atherosclerosis and coagulopathy, rheumatoid arthritis and tumour metastasis. Here, we describe some of the critical mechanisms for generating soluble GPVI from platelets, and future avenues for exploiting this unique platelet-specific receptor for diagnosis and/or disease prevention.

Platelet glycoprotein VI aids in local immunity during pneumonia-derived sepsis caused by gram-negative bacteria

Platelet collagen receptor glycoprotein VI (GPVI) and podoplanin receptor C-type lectin-like receptor 2 (CLEC2) are receptors implicated in platelet activation that both signal via an immunoreceptor tyrosine-based activation motif. Platelets are necessary for host defense and prevention of hemorrhage during sepsis, but the role of platelet GPVI and CLEC2 herein is unknown. To investigate this, we infected mice depleted of platelet GPVI or CLEC2 by antibody treatment or GPVI^-/- mice with the common human sepsis pathogen Klebsiella pneumoniae via the airways to induce pneumonia-derived sepsis. The GPVI ligand collagen and the CLEC2 ligand podoplanin were constitutively present in the lung, whereas the GPVI ligands fibrin and histone were induced during pneumonia. During late-stage infection, both mice depleted of GPVI and GPVI^-/- mice showed increased bacterial growth in lungs, and GPVI^-/- mice also showed increased bacterial growth in distant body sites. Despite higher bacterial loads, GPVI-depleted mice showed reduced platelet numbers, platelet activation, and platelet-leukocyte complex formation in the bronchoalveolar space. Consistently, in human whole blood, GPVI stimulation of platelets increased platelet-leukocyte complex formation and leukocyte activation, which was accompanied by enhanced phagocytosis of Klebsiella GPVI-depleted mice showed increased lung hemorrhage during infection, but not to the extent observed in platelet-depleted mice, and lung bleeding was not significantly different between GPVI^-/- and wild-type mice. CLEC2 depletion did not affect any of the responses during pneumonia. These results suggest that platelet GPVI, but not CLEC2, contributes to local host defense during pneumonia-derived sepsis by enhancing leukocyte function.

Proplatelet formation is selectively inhibited by collagen type I through Syk-independent GPVI signaling

Collagen receptors GPVI (also known as GP6) and integrin α2β1 are highly expressed on blood platelets and megakaryocytes, their immediate precursors. After vessel injury, subendothelial collagen becomes exposed and induces platelet activation to prevent blood loss. Collagen types I and IV are thought to have opposite effects on platelet biogenesis, directing proplatelet formation (PPF) towards the blood vessels to prevent premature release within the marrow cavity. We used megakaryocytes lacking collagen receptors or treated megakaryocytes with blocking antibodies, and could demonstrate that collagen-I-mediated inhibition of PPF is specifically controlled by GPVI. Other collagen types competed for binding and diminished the inhibitory signal, which was entirely dependent on receptor-proximal Src family kinases, whereas Syk and LAT were dispensable. Adhesion assays indicate that megakaryocyte binding to collagens is mediated by α2β1, and that collagen IV at the vascular niche might displace collagen I from megakaryocytes and thus contribute to prevention of premature platelet release into the marrow cavity and thereby directionally promote PPF at the vasculature.

FcγRIIB on liver sinusoidal endothelial cells is essential for antibody-induced GPVI ectodomain shedding in mice

The activating platelet collagen receptor glycoprotein VI (GPVI) is a promising antithrombotic target because of its central role in arterial thrombosis and its minor relevance for normal hemostasis. The receptor can be specifically targeted by antibodies and irreversibly downregulated in circulating platelets in vivo, resulting in long-term antithrombotic protection in mice. This GPVI immunodepletion predominantly occurs through ectodomain shedding, which is accompanied by a transient drop in peripheral platelet counts. Mechanistic studies on this targeted GPVI loss have been hampered because it cannot be reproduced in isolated platelets in vitro. Here we show that both the transient thrombocytopenia and GPVI ectodomain shedding depend on the Fc portion of the anti-GPVI antibody and its interaction with the inhibitory Fcγ receptor (FcγR)IIB. In wild-type, but not Fcgr2b(-/-) mice, anti-GPVI-opsonized platelets became transiently trapped in the liver followed by the appearance of the soluble GPVI ectodomain in the plasma. Depletion of Kupffer cells neither affected anti-GPVI-induced platelet accumulation nor GPVI shedding, demonstrating that the other major FcγRIIB-expressing cell type, liver sinusoidal endothelial cells, is required for both processes to occur. These results reveal a novel and unexpected function of hepatic FcγRIIB in the targeted downregulation of GPVI in vivo.

PI3K/Akt in platelet integrin signaling and implications in thrombosis

Blood platelets are anucleated circulating cells that play a critical role in hemostasis and are also implicated in arterial thrombosis, a major cause of death worldwide. The biological function of platelets strongly relies in their reactiveness to a variety of extracellular agonists that regulate their adhesion to extracellular matrix at the site of vascular injury and their ability to form rapidly growing cell aggregates. Among the membrane receptors expressed on the cell surface, integrins are crucial for both platelet activation, adhesion and aggregation. Integrin affinity for specific ligands is regulated by intracellular signaling pathways activated in stimulated platelets, and, once engaged, integrins themselves generate and propagate signals inside the cells to reinforce and consolidate platelet response and thrombus formation. Phosphatidylinositol 3-Kinases (PI3Ks) have emerged as crucial players in platelet activation, and they are directly implicated in the regulation of integrin function. This review will discuss the contribution of PI3Ks in platelet integrin signaling, focusing on the role of specific members of class I PI3Ks and their downstream effector Akt on both integrin inside-out and outside-in signaling. The contribution of the PI3K/Akt pathways stimulated by integrin engagement and platelet activation in thrombus formation and stabilization will also be discussed in order to highlight the possibility to target these enzymes in effective anti-thrombotic therapeutic strategies.

Targeted downregulation of platelet CLEC-2 occurs through Syk-independent internalization

Platelet aggregation at sites of vascular injury is not only essential for hemostasis, but may also cause acute ischemic disease states such as myocardial infarction or stroke. The hemi-immunoreceptor tyrosine-based activation motif-containing C-type lectinlike receptor 2 (CLEC-2) mediates powerful platelet activation through a Src- and spleen tyrosine kinase (Syk)-dependent tyrosine phosphorylation cascade. Thereby, CLEC-2 not only contributes to thrombus formation and stabilization but also plays a central role in blood-lymphatic vessel development, tumor metastasis, and prevention of inflammatory bleeding, making it a potential pharmacologic target to modulate these processes. We have previously shown that injection of the anti-CLEC-2 antibody, INU1, results in virtually complete immunodepletion of platelet CLEC-2 in mice, which is, however, preceded by a severe transient thrombocytopenia thereby limiting its potential therapeutic use. The mechanisms underlying this targeted CLEC-2 downregulation have remained elusive. Here, we show that INU1-induced CLEC-2 immunodepletion occurs through Src-family kinase-dependent receptor internalization in vitro and in vivo, presumably followed by intracellular degradation. In mice with platelet-specific Syk deficiency, INU1-induced CLEC-2 internalization/degradation was fully preserved whereas the associated thrombocytopenia was largely prevented. These results show for the first time that CLEC-2 can be downregulated from the platelet surface through internalization in vitro and in vivo and that this can be mechanistically uncoupled from the associated antibody-induced thrombocytopenia.

Hemostatic disorders in a JAK2V617F-driven mouse model of myeloproliferative neoplasm

Thrombosis is common in patients suffering from myeloproliferative neoplasm (MPN), whereas bleeding is less frequent. JAK2(V617F), the main mutation involved in MPN, is considered as a risk factor for thrombosis, although the direct link between the mutation and hemostatic disorders is not strictly established. We investigated this question using conditional JAK2(V617F) knock-in mice with constitutive and inducible expression of JAK2(V617F) in hematopoietic cells, which develop a polycythemia vera (PV)-like disorder evolving into myelofibrosis. In vitro, thrombosis was markedly impaired with an 80% decrease in platelet-covered surface, when JAK2(V617F) blood was perfused at arterial shear over collagen. JAK2(V617F) platelets presented only a moderate glycoprotein (GP) VI deficiency not responsible for the defective platelet accumulation. In contrast, a decreased proportion of high-molecular-weight von Willebrand factor multimers could reduce platelet adhesion. Accordingly, the tail bleeding time was prolonged. In the FeCl3-induced thrombosis model, platelet aggregates formed rapidly but were highly unstable. Interestingly, vessels were considerably dilated. Thus, mice developing PV secondary to constitutive JAK2(V617F) expression exhibit a bleeding tendency combined with the accelerated formation of unstable clots, reminiscent of observations made in patients. Hemostatic defects were not concomitant with the induction of JAK2(V617F) expression, suggesting they were not directly caused by the mutation but were rather the consequence of perturbations in blood and vessel homeostasis.

Targeting glycoprotein VI and the immunoreceptor tyrosine-based activation motif signaling pathway

Coronary artery thrombosis and ischemic stroke are often initiated by the disruption of an atherosclerotic plaque and consequent intravascular platelet activation. Thus, antiplatelet drugs are central in the treatment and prevention of the initial, and subsequent, vascular events. However, novel pharmacological targets for platelet inhibition remain an important goal of cardiovascular research because of the negative effect of existing antiplatelet drugs on primary hemostasis. One promising target is the platelet collagen receptor glycoprotein VI. Blockade or antibody-mediated depletion of this receptor in circulating platelets is beneficial in experimental models of thrombosis and thrombo-inflammatory diseases, such as stroke, without impairing hemostasis. In this review, we summarize the importance of glycoprotein VI and (hem)immunoreceptor tyrosine-based activation motif signaling in hemostasis, thrombosis, and thrombo-inflammatory processes and discuss the targeting strategies currently under development for inhibiting glycoprotein VI and its signaling.

Targeting GPVI as a novel antithrombotic strategy

While platelet activation is essential to maintain blood vessel patency and minimize loss of blood upon injury, untimely or excessive activity can lead to unwanted platelet activation and aggregation. Resultant thrombosis has the potential to block blood vessels, causing myocardial infarction or stroke. To tackle this major cause of mortality, clinical therapies that target platelet responsiveness (antiplatelet therapy) can successfully reduce cardiovascular events, especially in people at higher risk; however, all current antiplatelet therapies carry an increased probability of bleeding. This review will evaluate new and emerging targets for antithrombotics, focusing particularly on platelet glycoprotein VI, as blockade or depletion of this platelet-specific receptor conveys benefits in experimental models of thrombosis and thromboinflammation without causing major bleeding complications.

Platelet immunoreceptor tyrosine-based activation motif (ITAM) signaling and vascular integrity

Platelets are well-known for their critical role in hemostasis, that is, the prevention of blood loss at sites of mechanical vessel injury. Inappropriate platelet activation and adhesion, however, can lead to thrombotic complications, such as myocardial infarction and stroke. To fulfill its role in hemostasis, the platelet is equipped with various G protein-coupled receptors that mediate the response to soluble agonists such as thrombin, ADP, and thromboxane A2. In addition to G protein-coupled receptors, platelets express 3 glycoproteins that belong to the family of immunoreceptor tyrosine-based activation motif receptors: Fc receptor γ chain, which is noncovalently associated with the glycoprotein VI collagen receptor, C-type lectin 2, the receptor for podoplanin, and Fc receptor γII A, a low-affinity receptor for immune complexes. Although both genetic and chemical approaches have documented a critical role for platelet G protein-coupled receptors in hemostasis, the contribution of immunoreceptor tyrosine-based activation motif receptors to this process is less defined. Studies performed during the past decade, however, have identified new roles for platelet immunoreceptor tyrosine-based activation motif signaling in vascular integrity in utero and at sites of inflammation. The purpose of this review is to summarize recent findings on how platelet immunoreceptor tyrosine-based activation motif signaling controls vascular integrity, both in the presence and absence of mechanical injury.

Combined in vivo depletion of glycoprotein VI and C-type lectin-like receptor 2 severely compromises hemostasis and abrogates arterial thrombosis in mice

OBJECTIVE

Platelet inhibition is a major strategy to prevent acute ischemic cardiovascular and cerebrovascular events, which may, however, be associated with an increased bleeding risk. The (hem)immunoreceptor tyrosine activation motif-bearing platelet receptors, glycoprotein VI (GPVI) and C-type lectin-like receptor 2 (CLEC-2), might be promising antithrombotic targets because they can be depleted from circulating platelets by antibody treatment, leading to sustained antithrombotic protection, but only moderately increased bleeding times in mice.

APPROACH AND RESULTS

We investigated whether both (hem)immunoreceptor tyrosine activation motif-bearing receptors can be targeted simultaneously and what the in vivo consequences of such a combined therapeutic GPVI/CLEC-2 deficiency are. We demonstrate that isolated targeting of either GPVI or CLEC-2 in vivo does not affect expression or function of the respective other receptor. Moreover, simultaneous treatment with both antibodies resulted in the sustained loss of both GPVI and CLEC-2, while leaving other activation pathways intact. However, GPVI/CLEC-2-depleted mice displayed a dramatic hemostatic defect and profound impairment of arterial thrombus formation. Furthermore, a strongly diminished hemostatic response could also be reproduced in mice genetically lacking GPVI and CLEC-2.

CONCLUSIONS

These results demonstrate that GPVI and CLEC-2 can be simultaneously downregulated in platelets in vivo and reveal an unexpected functional redundancy of the 2 receptors in hemostasis and thrombosis. These findings may have important implications of the potential use of anti-GPVI and anti-CLEC-2-based agents in the prevention of thrombotic diseases.

The SLAM family member CD84 is regulated by ADAM10 and calpain in platelets

BACKGROUND AND OBJECTIVE

Ectodomain shedding is a major mechanism to modulate platelet receptor signaling and to downregulate platelet reactivity. Proteins of the a disintegrin and metalloproteinase (ADAM) family are implicated in the shedding of various platelet receptors. The signaling lymphocyte activation molecule (SLAM) family receptor CD84 is highly expressed in platelets and immune cells, but its role in platelet physiology is not well explored. Because of its ability to form homodimers, CD84 has been suggested to mediate contact-dependent signaling and contribute to thrombus stability. However, nothing is known about the cellular regulation of CD84.

METHODS

We studied the regulation of CD84 in murine platelets by biochemical approaches and use of three different genetically modified mouse lines. Regulation of CD84 in human platelets was studied using inhibitors and biochemical approaches.

RESULTS

We show that CD84 is cleaved from the surface of human and murine platelets in response to different shedding inducing agents and platelet receptor agonists. CD84 downregulation occurs through ectodomain-shedding and intracellular cleavage. Studies in transgenic mice identified ADAM10 as the principal sheddase responsible for CD84 cleavage, whereas ADAM17 was dispensable. Western blot analyses revealed calpain-mediated intracellular cleavage of the CD84 C-terminus, occurring simultaneously with, but independently of, ectodomain shedding. Furthermore, analysis of plasma and serum samples from transgenic mice demonstrated that CD84 is constitutively shed from the platelet surface by ADAM10 in vivo.

CONCLUSIONS

These results reveal a dual regulation mechanism for platelet CD84 by simultaneous extra- and intracellular cleavage that may modulate platelet-platelet and platelet-immune cell interactions.

The future of glycoprotein VI as an antithrombotic target

The treatment of acute coronary syndromes has been considerably improved in recent years with the introduction of highly efficient antiplatelet drugs. However, there are still significant limitations: the recurrence of adverse vascular events remains a problem, and the improvement in efficacy is counterbalanced by an increased risk of bleeding, which is of particular importance in patients at risk of stroke. One of the most attractive targets for the development of new molecules with potential antithrombotic activity is platelet glycoprotein (GP)VI, because its blockade appears to ideally combine efficacy and safety. This review summarizes current knowledge on GPVI regarding its structure, its function, and its role in physiologic hemostasis and thrombosis. Strategies for inhibiting GPVI are presented, and evidence of the antithrombotic efficacy and safety of GPVI antagonists is provided.

Desialylation accelerates platelet clearance after refrigeration and initiates GPIbα metalloproteinase-mediated cleavage in mice

When refrigerated platelets are rewarmed, they secrete active sialidases, including the lysosomal sialidase Neu1, and express surface Neu3 that remove sialic acid from platelet von Willebrand factor receptor (VWFR), specifically the GPIbα subunit. The recovery and circulation of refrigerated platelets is greatly improved by storage in the presence of inhibitors of sialidases. Desialylated VWFR is also a target for metalloproteinases (MPs), because GPIbα and GPV are cleaved from the surface of refrigerated platelets. Receptor shedding is inhibited by the MP inhibitor GM6001 and does not occur in Adam17(ΔZn/ΔZn) platelets expressing inactive ADAM17. Critically, desialylation in the absence of MP-mediated receptor shedding is sufficient to cause the rapid clearance of platelets from circulation. Desialylation of platelet VWFR therefore triggers platelet clearance and primes GPIbα and GPV for MP-dependent cleavage.

Design and reshaping of an scFv directed against human platelet glycoprotein VI with diagnostic potential

Blood platelets play a key role in physiological hemostasis and in thrombosis. As a consequence, platelet functional analysis is widely used in the diagnosis of hemorrhagic disorders as well as in the evaluation of thrombosis risks and of the efficacy of antithrombotics. Glycoprotein (GP) VI is a platelet-specific collagen-signaling receptor. Clinical studies suggest that increased GPVI expression is associated with a risk of arterial thrombosis. Conversely, GPVI deficiencies have been identified in patients with defective platelet responses to collagen. Currently, there is no standard test available for measuring GPVI expression, essentially because antibodies usually cross-link GPVI upon binding, leading to platelet activation and consecutive changes in GPVI expression. Here, we designed a recombinant monovalent antibody fragment (scFv) derived from an anti-GPVI monoclonal IgG, 3J24, with the characteristics required to analyze GPVI expression. Guided by in silico modeling and V-KAPPA chain analysis, a Protein L (PpL) recognition pattern was engineered in the scFv, making possible its purification and detection using PpL conjugates. The PpL affinity-purified scFv is functional. It retains GPVI-binding specificity and allows detection of platelet surface-expressed GPVI without inducing platelet activation. In conclusion, the reshaped scFv may be very useful in the development of diagnostic approaches.

Platelet adhesion and activation mechanisms in arterial thrombosis and ischaemic stroke

Platelet adhesion, activation and aggregation on the exposed subendothelial extracellular matrix (ECM) are essential for haemostasis, but may also lead to occlusion of diseased vessels. Binding of the glycoprotein (GP)Ib-V-IX complex to immobilised von Willebrand factor (VWF) initiates adhesion of flowing platelets to the ECM, and thereby enables the collagen receptor GPVI to interact with its ligand and to mediate platelet activation. This process is reinforced by locally produced thrombin and platelet-derived secondary mediators, such as adenosine diphosphate (ADP) and thromboxane A(2) (TxA(2)). Together, these events promote a shift of β1 and β3 integrins from a low to a high affinity state for their ligands through 'inside-out' signalling allowing firm platelet adhesion and aggregation. Formed platelet aggregates are stabilised by fibrin formation and signalling events between adjacent platelets involving multiple platelet receptors, such as the newly discovered C-type lectin-like receptor 2 (CLEC-2). While occlusive thrombus formation is the principal pathogenic event in myocardial infarction, the situation is more complex in ischaemic stroke where infarct development often progresses despite sustained early reperfusion of previously occluded major intracranial arteries, a process referred to as 'reperfusion injury'. Increasing experimental evidence now suggests that early platelet adhesion and activation events, orchestrate a 'thrombo-inflammatory' cascade in this setting, whereas platelet aggregation and thrombus formation are not required. This review summarises recent developments in understanding the principal platelet adhesion receptor systems with a focus on their involvement in arterial thrombosis and ischaemic stroke models.

Proteolysis of platelet receptors in humans and other species

In the past 5 years, metalloproteinase-mediated ectodomain shedding of platelet receptors has emerged as a new mechanism for modulating platelet function. By regulating surface expression of the platelet-specific receptors, glycoprotein (GP)VI that binds collagen, and GPIbalpha (the major ligand-binding subunit of the GPIb-IX-V complex) that binds von Willebrand factor (VWF) and other procoagulant and proinflammatory ligands, shedding not only irreversibly downregulates GPVI/GPIbalpha function, but generates proteolytic fragments that might be unique biomarkers or modulators in plasma. This is potentially significant because GPVI and GPIbalpha are involved in initiating thrombotic diseases such as heart attack and stroke, as well as autoimmune diseases where anti-platelet antibodies result in thrombocytopenia. Altered expression levels of GPIbalpha/GPVI are associated with both thrombotic propensity and platelet aging, suggesting an additional role in platelet clearance. Although emerging data are elucidating molecular mechanisms underlying GPIbalpha/GPVI shedding, evidence for the functional consequences of shedding in vivo, either clinically or in animal models, is far more limited. Here we consider recent published evidence for GPVI or GPIbalpha shedding in humans, nonhuman primates and mice, and whether conservation of sheddase cleavage sites across species points to a functional role for metalloproteolytic shedding in vivo.

GPVI and the not so eager cleaver

Platelets control their responsiveness, in part, by shedding adhesion and signaling receptors from their surface. The molecular mechanism by which this occurs, however,is incompletely understood. In this issue of Blood, Bender and colleagues make judicious use of mice genetically deficient in selected candidate proteases to shed new light on the unexpected complexity of ectodomain shedding.

PTEN regulates collagen-induced platelet activation

Phosphatidylinositol 3-kinase (PI3K) has been shown to play an important role in collagen-induced platelet activation, but the role(s) of PTEN, a major regulator of the PI3K/Akt signaling pathway, has not been examined in platelets. Here, we report that Pten−/− mouse blood contains 25% more platelets than Pten+/+ blood and that PTEN deficiency significantly shortened the bleeding time, increased the sensitivity of platelets to collagen-induced activation and aggregation, and enhanced phosphorylation of Akt at Ser473 in response to collagen. Furthermore, we found that PP2, and the combination of apyrase, indomethacin + 1B5, respectively, inhibited collagen-induced aggregation in both PTEN+/+ and PTEN−/− platelets. In contrast, LY294002 (a PI3K inhibitor) prevented the aggregation of PTEN+/+, but not PTEN−/−, platelets. Therefore, PTEN apparently regulates collagen-induced platelet activation through PI3K/Akt-dependent and -independent signaling pathways.

Differentially regulated GPVI ectodomain shedding by multiple platelet-expressed proteinases

Glycoprotein VI (GPVI) mediates platelet activation on exposed subendothelial collagens at sites of vascular injury and thereby contributes to normal hemostasis, but also to the occlusion of diseased vessels in the setting of myocardial infarction or stroke. GPVI is an attractive target for antithrombotic therapy, particularly because previous studies have shown that anti-GPVI antibodies induce irreversible down-regulation of the receptor in circulating platelets by internalization and/or ectodomain shedding. Metalloproteinases of the a disintegrin and metalloproteinase (ADAM) family have been proposed to mediate this ectodomain shedding, but direct evidence for this is lacking. Here, we studied GPVI shedding in vitro and in vivo in newly generated mice with a megakaryocyte-specific ADAM10 deficiency and in Adam17(ex/ex) mice, which lack functional ADAM17. We demonstrate that GPVI cleavage in vitro can occur independently through either ADAM10 or ADAM17 in response to distinct stimuli. In contrast, antibody (JAQ1)-induced GPVI shedding in vivo occurred in mice lacking both ADAM10/ADAM17 in their platelets, suggesting the existence of a third GPVI cleaving platelet enzyme. This was supported by in vitro studies on ADAM10/ADAM17 double-deficient platelets. These results reveal that ectodomain shedding of GPVI can be mediated through multiple differentially regulated platelet-expressed proteinases with obvious therapeutic implications.

Arterial thrombosis: relevance of a model with two levels of severity assessed by histologic, ultrastructural and functional characterization

BACKGROUND

We previously described a model of laser-induced thrombosis in mesenteric arterioles with superficial and deep levels of injury producing a transient thrombus resolving within 2 min and a larger almost occlusive thrombus, respectively. Both types of lesion were sensitive to platelet GPIIb-IIIa and P2Y(12) inhibition, whereas only deep injuries were sensitive to thrombin blockade.

OBJECTIVE

The aim of the present study was to use histologic methods and electron and intravital microscopy to characterize the lesions and thrombi and to extend our knowledge of the sensitivity of this model to genetic and pharmacologic inhibition.

RESULTS

A superficial injury was found to detach the endothelial cells and expose a collagen III- and IV-rich subendothelium where platelets could adhere. Tissue factor and fibrin were not detected. Deeper penetration of the external elastic lamina occurred in deep injuries, with exposure of collagen I, III and IV. Here the thrombus was composed of platelets exhibiting a decreasing gradient of degranulation from the deepest lesion area to the surface. Fibrin was found close to the most activated platelets. Consistently, glycoprotein VI (GPVI)-collagen and GPIb-von Willebrand factor (VWF) interactions were found to be critical in superficial injuries. After deep lesion, thrombus formation was modestly reduced in GPVI-immunodepleted mice and still strongly inhibited in VWF(-/-) mice. Combined hirudin infusion and GPVI depletion further inhibited thrombosis after deep injury.

CONCLUSIONS

This study confirms the feasibility of inducing arterial thrombosis with distinct levels of severity and establishes the central roles of collagen and VWF in thrombus formation after superficial injury. Collagen, VWF and thrombin all appear to contribute to thrombosis after deep arterial lesion.

An acquired inhibitor to the GPVI platelet collagen receptor in a patient with lupus nephritis

BACKGROUND

GPVI is a major platelet collagen signaling receptor. In rare cases of immune thrombocytopenic purpura (ITP), autoantibodies to GPVI result in receptor shedding.

OBJECTIVES

To investigate a possible pathogenic role of plasma anti-GPVI antibody located in a woman with lupus nephritis.

METHODS

Measured were (i) platelet aggregation to collagen and convulxin, (ii) platelet GPVI expression (flow cytometry and western blotting), (iii) plasma soluble GPVI (sGPVI, dual antibody ELISA), and (iv) plasma anti-GPVI antibody (ELISA using recombinant sGPVI).

RESULTS

In 2006 and early 2007, the patient had a normal platelet count but a virtual absence of platelet aggregation to collagen and convulxin. Her platelets responded normally to other agonists including cross-linking ITAM-dependent FcgammaRIIA by monoclonal antibody, IV.3. Flow cytometry and western blotting showed a platelet deficiency of GPVI. Plasma sGPVI levels were undetectable whereas ELISA confirmed the presence of anti-GPVI antibody. Sequencing revealed a normal GPVI cDNA structure. The patient's plasma and the isolated IgG3 fraction activated and induced GPVI shedding from normal platelets. A deteriorating clinical condition led to increasingly strict immunosuppressive therapy. This was globally associated with a fall in plasma anti-GPVI titres, the restoration of platelet GPVI and the convulxin response, and the loss of her nephrotic syndrome.

CONCLUSIONS

Our results show that this patient acquired a potent anti-GPVI IgG3 antibody with loss of GPVI and collagen-related platelet function. Further studies are required to determine whether anti-GPVI antibodies occur in other lupus patients with nephritis.

Presence of platelet-associated anti-glycoprotein (GP)VI autoantibodies and restoration of GPVI expression in patients with GPVI deficiency

BACKGROUND

Glycoprotein (GP)VI deficiency is a rare platelet disorder with a mild bleeding tendency. However, its pathophysiology remains unclear.

OBJECTIVES

We characterized a novel GPVI-deficient patient with immune thrombocytopenic purpura and searched for the presence of anti-GPVI autoantibodies in this and another patient with GPVI deficiency.

METHODS AND RESULTS

A 12-year-old Japanese girl (case 1) with moderate thrombocytopenia and mild bleeding showed selectively impaired collagen-induced platelet aggregation. Flow cytometric analysis indicated that the patient had a defect in the expression of GPVI-FcRgamma. An eluate of her platelet-associated IgG contained anti-alpha(IIb)beta3 autoantibodies. Moreover, using GPVI-FcRgamma-transfected cells, we unexpectedly identified anti-GPVI antibodies against the soluble ectodomain of GPVI in the eluate, despite the patient's GPVI deficiency. In contrast, anti-GPVI antibodies were not detectable in her plasma. In another case of GPVI deficiency (case 2) without detectable plasma anti-GPVI antibodies, we again detected platelet-associated anti-GPVI antibodies. In a 2-year follow-up of case 1, the platelet count increased to within the normal range and the bleeding tendency improved. Interestingly, GPVI was again expressed on her platelets, in association with a decrease in the relative amount of anti-GPVI antibodies.

CONCLUSIONS

This is the first demonstration of platelet-associated anti-GPVI antibodies in GPVI-deficient subjects, in one case with spontaneous restoration of GPVI expression. These results strongly suggest an autoimmune mechanism in GPVI deficiency.

CLEC-2 is an essential platelet-activating receptor in hemostasis and thrombosis

Damage to the integrity of the vessel wall leads to exposure of the subendothelial extracellular matrix (ECM), triggering platelet activation and aggregation. This process is essential for primary hemostasis but it may also lead to arterial thrombosis. Although the mechanisms underlying platelet activation on the ECM are well explored, it is less clear which receptors mediate cellular activation in a growing thrombus. Here we studied the role of the recently identified C-type lectin-like receptor 2 (CLEC-2) in this process. We show that anti-CLEC-2 antibody treatment of mice leads to complete and highly specific loss of CLEC-2 in circulating platelets for several days. CLEC-2-deficient platelets displayed normal adhesion under flow, but subsequent aggregate formation was severely defective in vitro and in vivo. As a consequence, CLEC-2 deficiency was associated with increased bleeding times and profound protection from occlusive arterial thrombus formation. These results reveal an essential function of CLEC-2 in hemostasis and thrombosis.

The Y's that bind: negative regulators of Src family kinase activity in platelets

Members of the Src family of protein tyrosine kinases play important roles in platelet adhesion, activation, and aggregation. The purpose of this review is to summarize current knowledge regarding how Src family kinase activity is regulated in general, to describe what is known about mechanisms underlying SFK activation in platelets, and to discuss platelet proteins that contribute to SFK inactivation, particularly those that use phosphotyrosine-containing sequences to recruit phosphatases and kinases to sites of SFK activity.

Absence of collagen-induced platelet activation caused by compound heterozygous GPVI mutations

The glycoprotein VI (GPVI)/FcRgamma complex is a key receptor for platelet activation by collagen. We describe, for the first time, 2 genetic abnormalities in one patient. This 10-year-old girl presented ecchymoses since infancy, a prolonged bleeding time despite a normal platelet count and no antiplatelet antibodies. Collagen-induced platelet activation was null, whereas GPVI quantification by flow cytometry evidenced an incomplete deficiency. Immunoblotting showed an abnormal migration of residual GPVI, and no FcRgamma defect. GPVI DNA sequencing revealed (1) an R38C mutation in exon 3 of one allele and (2) an insertion of 5 nucleotides in exon 4 of the other allele, leading to a premature nonsense codon and absence of the corresponding mRNA. Introduction of the R38C mutation into recombinant GPVI-Fc resulted in abnormal protein migration and a loss of collagen binding. Thus, this composite genetic GPVI deficiency and dysfunction cause absence of platelet responses to collagen and a mild bleeding phenotype.

Regulation of platelet glycoprotein VI (GPVI) surface expression and of soluble GPVI in patients with atrial fibrillation (AF) and acute coronary syndrome (ACS)

BACKGROUND

The platelet collagen receptor glycoprotein VI (GPVI) mediates platelet adhesion to subendothelial matrix and thrombus formation in acute coronary syndrome (ACS). This study examined patients with both ACS and stable coronary artery disease (CAD), which presented with atrial fibrillation (AF) and sinus rhythm (SR).

METHODS AND RESULTS

We evaluated 992 patients with acute or stable CAD, and determined platelet surface expression of GPVI using flow cytometry. Seventy-eight patients presented with nonvalvular persistent AF. After 1:1 propensity score matching 156 matched cases with 78 pairs were obtained. Patients with AF and ACS showed a significantly decreased GPVI expression compared to patients with ACS and SR, whereas patients with stable angina pectoris (SA) presented with low level activation and no significant difference between SR and AF [mean fluorescence intensity (MFI) for ACS (SR Vs. AF): 20 +/- 6.3 Vs. 17.7 +/- 4.4; P = 0.023; SA (SR Vs. AF): 18.8 +/- 9.4 Vs. 18.1 +/- 6.1; P = 0.649]. In contrast, soluble GPVI was increased in ACS and AF accordingly [plasma GPVI (ng/ml) for ACS (SR Vs. AF): 1.4 +/- 0.8 Vs. 1.9 +/- 1.1; P = 0.038; SA (SR Vs. AF): 0.9 +/- 0.4 Vs. 1.1 +/- 0.5; P = 0.127].

CONCLUSION

Platelet GPVI surface expression is decreased in patients with AF and ACS compared to patients with SR and ACS. Nonvalvular AF is related to indices of chronic platelet activation and might be responsible for a down-regulation of GPVI receptor density on platelets, while soluble GPVI was increased in ACS and AF accordingly.

A novel antiplatelet antibody therapy that induces cAMP-dependent endocytosis of the GPVI/Fc receptor gamma-chain complex

Platelet adhesion to vascular subendothelium, mediated in part by interactions between collagen and glycoprotein VI (GPVI) complexed with Fc receptor gamma-chain, is crucial for thrombus formation. Antiplatelet therapy benefits patients with various thrombotic and ischemic diseases, but the safety and efficacy of existing treatments are limited. Recent data suggest GPVI as a promising target for a novel antiplatelet therapy, for example, GPVI-specific Abs that deplete GPVI from the surface of platelets. Here, we characterized GPVI-specific auto-Abs (YA-Abs) from the first reported patient with ongoing platelet GPVI deficiency caused by the YA-Abs. To obtain experimentally useful human GPVI-specific mAbs with characteristics similar to YA-Abs, we generated human GPVI-specific mouse mAbs and selected 2 representative mAbs, mF1201 and mF1232, whose binding to GPVI was inhibited by YA-Abs. In vitro, mF1201, but not mF1232, induced human platelet activation and GPVI shedding, and mF1232 inhibited collagen-induced human platelet aggregation. Administration of mF1201 and mF1232 to monkeys caused GPVI immunodepletion with and without both significant thrombocytopenia and GPVI shedding, respectively. When a human/mouse chimeric form of mF1232 (cF1232) was labeled with a fluorescent endocytosis probe and administered to monkeys, fluorescence increased in circulating platelets and surface GPVI was lost. Loss of platelet surface GPVI mediated by cF1232 was successfully reproduced in vitro in the presence of a cAMP-elevating agent. Thus, we have characterized cAMP-dependent endocytosis of GPVI mediated by a human GPVI-specific mAb as what we believe to be a novel antiplatelet therapy.

Compromised ITAM-based platelet receptor function in a patient with immune thrombocytopenic purpura

BACKGROUND

Receptors on platelets that contain immunoreceptor tyrosine-based activation motifs (ITAMs) include collagen receptor glycoprotein (GP) VI, and FcgammaRIIa, a low affinity receptor for immunoglobulin (Ig) G.

OBJECTIVES

We examined the function of GPVI and FcgammaRIIa in a patient diagnosed with immune thrombocytopenic purpura (ITP) who had unexplained pathological bruising despite normalization of the platelet count with treatment.

METHODS AND RESULTS

Patient platelets aggregated normally in response to ADP, arachadonic acid and epinephrine, but not to GPVI agonists, collagen or collagen-related peptide, or to FcgammaRII-activating monoclonal antibody (mAb) 8.26, suggesting ITAM receptor dysfunction. Plasma contained an anti-GPVI antibody by MAIPA and aggregated normal platelets. Aggregating activity was partially (approximately 60%) blocked by FcgammaRIIa-blocking antibody, IV.3, and completely blocked by soluble GPVI ectodomain. Full-length GPVI on the patient platelet surface was reduced to approximately 10% of normal levels, and a approximately 10-kDa GPVI cytoplasmic tail remnant and cleaved FcgammaRIIa were detectable by western blot, indicating platelet receptor proteolysis. Plasma from the patient contained approximately 150 ng mL(-1) soluble GPVI by ELISA (normal plasma, approximately 15 ng mL(-1)) and IgG purified from patient plasma caused FcgammaRIIa-mediated, EDTA-sensitive cleavage of both GPVI and FcgammaRIIa on normal platelets.

CONCLUSIONS

In ITP patients, platelet autoantibodies can curtail platelet receptor function. Platelet ITAM receptor dysfunction may contribute to the increased bleeding phenotype observed in some patients with ITP.

Platelet receptor redox regulation

Several recent findings point to an important role for redox regulation of platelet responses to collagen involving the receptor, glycoprotein (GP)VI. First, the antioxidant dietary compound, quercetin, was shown to inhibit GPVI-dependent platelet activation and signaling responses to collagen. Second, collagen increased platelet production of the oxygen radical, superoxide anion (O2-), mediated by the multi-subunit enzyme nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) oxidase. In that case, O2- was implicated in regulating not initial aggregation, but collagen-induced thrombus stabilization involving release of ADP. Third, our laboratory showed that an unpaired thiol in the GPVI cytoplasmic tail undergoes rapid oxidation to form GPVI homodimers following ligand binding, preceding GPVI signaling and ectodomain metalloproteolysis, and indicating formation of an oxidative submembranous environment in activated platelets. This review examines receptor/redox regulation in other cells, and relevance to the pathophysiological function of GPVI and other platelet receptors initiating thrombus formation in haemostasis or thrombotic diseases such as heart attack and stroke.

Ex vivo inhibition of thrombus formation by an anti-glycoprotein VI Fab fragment in non-human primates without modification of glycoprotein VI expression

OBJECTIVES

Glycoprotein (GP)VI is an attractive target for the development of new antithrombotic drugs. Its deficiency protects animals in several models of thrombosis, arterial stenosis and ischemia--reperfusion while inducing no major bleeding tendency. The Fab fragment of one anti-GPVI monoclonal antibody (9O12.2) inhibits all GPVI functions in vitro. The aim of this study was to determine the ex vivo effects of 9O12.2 Fab on hemostasis, coagulation and thrombosis in non-human primates.

METHODS AND RESULTS

Blood samples were collected from cynomolgus monkeys before and after (30, 90 and 150 min, 1 and 7 days) a bolus injection of 9O12.2 Fab (4 mg kg(-1)) or vehicle. Platelet counts and coagulation tests (prothrombin time, activated partial thromboplastin time) were not modified following Fab injection. The PFA-100 closure time increased during the first hours and returned to initial values on day + 1. Platelet-bound Fab was detected from 30 min to 24 h after Fab injection without GPVI depletion at any time. Collagen-induced platelet aggregation was selectively and fully inhibited at 30 min. Thrombus formation on collagen in flowing whole blood (1500 s(-1)) was delayed and decreased, and collagen-induced or tissue factor-induced thrombin generation in platelet-rich plasma was profoundly inhibited.

CONCLUSION

The anti-GPVI 9O12.2 Fab inhibits thrombus formation ex vivo in non-human primates with a composite effect on platelet activation and thrombin generation in the absence of GPVI depletion.

Platelet glycoprotein VI-related clinical defects

Human patients with defects associated with the platelet collagen receptor, glycoprotein (GP)VI, are rare and usually described as having a mild bleeding disorder. However, here we review clinical profiles of patients with familial or acquired GPVI defects, revealing the bleeding defect is often severe and associated with immune dysfunction. GPVI is a member of the immunoreceptor family, and co-expressed on platelets with Fc receptor gamma-chain (FcRgamma). Ligand binding to GPVI leads to activation of platelet integrins, in particular alpha(IIb)beta(3) that mediates platelet aggregation; and activation of endogenous platelet metalloproteinases resulting in ectodomain shedding and release of a soluble GPVI fragment. Increasing evidence supports the functional importance of GPVI/FcRgamma in thrombus formation at arterial shear rates, and expression levels of platelet GPVI may be a marker of thrombotic risk. Over the past 20 years, patients have been reported with GPVI-related defects involving: (i) an acquired deficiency, resulting from (a) anti-GPVI autoantibodies or (b) other causes; or (ii) a congenital deficiency, where (c) GPVI is not expressed or (d) is expressed in a dysfunctional form with defective signalling to alpha(IIb)beta(3). Clinical consequences of GPVI-related defects may be uniquely informative about the role of platelet GPVI in health and disease.