Does fibrin(ogen) bind to monomeric or dimeric GPVI, or not at all?

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

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSION

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

Glycoprotein VI interplay with fibrin(ogen) in thrombosis

Platelets play a central role in the arrest of bleeding. The ability of platelets to engage with extracellular matrix proteins of the subendothelium has long been recognized as a pivotal platelet attribute, underpinning adequate hemostasis. The propensity of platelets to rapidly bind and functionally respond to collagen was one of the earliest documented events in platelet biology. The receptor primarily responsible for mediating platelet/collagen responses was identified as glycoprotein (GP) VI and successfully cloned in 1999. Since that time, this receptor has held the attention of many research groups, and through these efforts, we now have an excellent understanding of the roles of GPVI as a platelet- and megakaryocyte-specific adheso-signaling receptor in platelet biology. GPVI is considered a viable antithrombotic target, as data obtained from groups across the world is consistent with GPVI being less involved in physiological hemostatic processes but participating in arterial thrombosis. This review will highlight the key aspects of GPVI contributions to platelet biology and concentrate on the interaction with recently identified ligands, with a focus on fibrin and fibrinogen, discussing the role of these interactions in the growth and stability of thrombi. We will also discuss important therapeutic developments that target GPVI to modulate platelet function while minimizing bleeding outcomes.

Characterizing the binding of glycoprotein VI with nanobody 35 reveals a novel monomeric structure of glycoprotein VI where the conformation of D1+D2 is independent of dimerization

BACKGROUND

The platelet-signaling receptor glycoprotein VI (GPVI) is a promising antithrombotic target. We have previously raised a series of high-affinity nanobodies (Nbs) against GPVI and identified Nb2, Nb21, and Nb35 as potent GPVI inhibitors. The Nb2 binding site has been mapped to the D1 domain, which is directly adjacent to the CRP binding site. Ligand-binding complementary determining region 3 has only 15% conservation between all 3 Nbs.

OBJECTIVES

To map the binding sites of Nb21 and Nb35 on GPVI.

METHODS

We determined the X-ray crystal structure of the D1 and D2 extracellular domains of the GPVI-Nb35 complex. We then looked at the effects of various GPVI mutations on the ability of Nbs to inhibit collagen binding and GPVI signaling using surface binding assays and transfected cell lines.

RESULTS

The crystal structure of GPVI bound to Nb35 was solved. GPVI was present as a monomer, and the D1+D2 conformation was comparable to that in the dimeric structure. Arg46, Tyr47, and Ala57 are common residues on GPVI targeted by both Nb2 and Nb35. Mutating Arg46 to an Ala abrogated the ability of Nb2, Nb21, and Nb35 to inhibit collagen-induced GPVI signaling and blocked the binding of all 3 Nbs. In addition, Arg60 was found to reduce Nb21 inhibition but not the inhibition Nb2 or Nb35.

CONCLUSIONS

These findings reveal key residues involved in the high-affinity binding of GPVI inhibitors and negate the idea that GPVI dimerization induces a conformational change required for ligand binding.

Posttranslational modifications of platelet adhesion receptors

Platelets play a key role in normal hemostasis, whereas pathological platelet adhesion is involved in various cardiovascular events. The underlying cause in cardiovascular events involves plaque rupture leading to subsequent platelet adhesion, activation, release, and eventual thrombosis. Traditional antithrombotic drugs often target the signal transduction process of platelet adhesion receptors by influencing the synthesis of some key molecules, and their effects are limited. Posttranslational modifications (PTMs) of platelet adhesion receptors increase the functional diversity of the receptors and affect platelet physiological and pathological processes. Antithrombotic drugs targeting PTMs of platelet adhesion receptors may represent a new therapeutic idea. In this review, various PTMs, including phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, lipidation, and proteolysis, of three platelet adhesion receptors, glycoprotein Ib-IX-V (GPIb-IX-V), glycoprotein VI (GPVI), and integrin α_IIbβ_3, are reviewed. It is important to comprehensively understand the PTMs process of platelet adhesion receptors.

Roles of Focal Adhesion Kinase PTK2 and Integrin αIIbβ3 Signaling in Collagen- and GPVI-Dependent Thrombus Formation under Shear

Glycoprotein (GP)VI and integrin αIIbβ3 are key signaling receptors in collagen-dependent platelet aggregation and in arterial thrombus formation under shear. The multiple downstream signaling pathways are still poorly understood. Here, we focused on disclosing the integrin-dependent roles of focal adhesion kinase (protein tyrosine kinase 2, PTK2), the shear-dependent collagen receptor GPR56 (ADGRG1 gene), and calcium and integrin-binding protein 1 (CIB1). We designed and synthetized peptides that interfered with integrin αIIb binding (pCIB and pCIBm) or mimicked the activation of GPR56 (pGRP). The results show that the combination of pGRP with PTK2 inhibition or of pGRP with pCIB > pCIBm in additive ways suppressed collagen- and GPVI-dependent platelet activation, thrombus buildup, and contraction. Microscopic thrombus formation was assessed by eight parameters (with script descriptions enclosed). The suppressive rather than activating effects of pGRP were confined to blood flow at a high shear rate. Blockage of PTK2 or interference of CIB1 no more than slightly affected thrombus formation at a low shear rate. Peptides did not influence GPVI-induced aggregation and Ca2+ signaling in the absence of shear. Together, these data reveal a shear-dependent signaling axis of PTK2, integrin αIIbβ3, and CIB1 in collagen- and GPVI-dependent thrombus formation, which is modulated by GPR56 and exclusively at high shear. This work thereby supports the role of PTK2 in integrin αIIbβ3 activation and signaling.

Targeting of a Conserved Epitope in Mouse and Human GPVI Differently Affects Receptor Function

Glycoprotein (GP) VI is the major platelet collagen receptor and a promising anti-thrombotic target. This was first demonstrated in mice using the rat monoclonal antibody JAQ1, which completely blocks the Collagen-Related Peptide (CRP)-binding site on mouse GPVI and efficiently inhibits mouse platelet adhesion, activation and aggregation on collagen. Here, we show for the first time that JAQ1 cross-reacts with human GPVI (huGPVI), but not with GPVI in other tested species, including rat, rabbit, guinea pig, swine, and dog. We further demonstrate that JAQ1 differently modulates mouse and human GPVI function. Similar to its effects on mouse GPVI (mGPVI), JAQ1 inhibits CRP-induced activation in human platelets, whereas, in stark contrast to mouse GPVI, it does not inhibit the adhesion, activation or aggregate formation of human platelets on collagen, but causes instead an increased response. This effect was also seen with platelets from newly generated human GPVI knockin mice (hGP6^tg/tg). These results indicate that the binding of JAQ1 to a structurally conserved epitope in GPVI differently affects its function in human and mouse platelets.

Procoagulant platelet sentinels prevent inflammatory bleeding through GPIIBIIIA and GPVI

Impairment of vascular integrity is a hallmark of inflammatory diseases. We recently reported that single immune-responsive platelets migrate and re-position themselves to sites of vascular injury to prevent bleeding. However, it remains unclear how single platelets preserve vascular integrity once encountering endothelial breaches. Here we demonstrate by intravital microscopy combined with genetic mouse models that procoagulant activation (PA) of single platelets and subsequent recruitment of the coagulation cascade are crucial for the prevention of inflammatory bleeding. Using a novel lactadherin-based compound we detect phosphatidylserine (PS)-positive procoagulant platelets in the inflamed vasculature. We identify exposed collagen as the central trigger arresting platelets and initiating subsequent PA in a CypD- and TMEM16F-dependent manner both in vivo and in vitro. Platelet PA promotes binding of the prothrombinase complex to the platelet membrane, greatly enhancing thrombin activity resulting in fibrin formation. PA of migrating platelets is initiated by co-stimulation via integrin αIIbβ3 (GPIIBIIIA)/Gα13-mediated outside-in-signaling and GPVI signaling, leading to an above-threshold intracellular calcium release. This effectively targets the coagulation cascade to breaches of vascular integrity identified by patrolling platelets. Platelet-specific genetic loss of either CypD or TMEM16F as well as combined blockade of platelet GPIIBIIIA and GPVI reduce platelet PA in vivo and aggravate pulmonary inflammatory hemorrhage. Our findings illustrate a novel role of procoagulant platelets in the prevention of inflammatory bleeding and provide evidence that PA of patrolling platelet sentinels effectively targets and confines activation of coagulation to breaches of vascular integrity.

Prothrombotic Phenotype in COVID-19: Focus on Platelets

COVID-19 infection is associated with a broad spectrum of presentations, but alveolar capillary microthrombi have been described as a common finding in COVID-19 patients, appearing as a consequence of a severe endothelial injury with endothelial cell membrane disruption. These observations clearly point to the identification of a COVID-19-associated coagulopathy, which may contribute to thrombosis, multi-organ damage, and cause of severity and fatality. One significant finding that emerges in prothrombotic abnormalities observed in COVID-19 patients is that the coagulation alterations are mainly mediated by the activation of platelets and intrinsically related to viral-mediated endothelial inflammation. Beyond the well-known role in hemostasis, the ability of platelets to also release various potent cytokines and chemokines has elevated these small cells from simple cell fragments to crucial modulators in the blood, including their inflammatory functions, that have a large influence on the immune response during infectious disease. Indeed, platelets are involved in the pathogenesis of acute lung injury also by promoting NET formation and affecting vascular permeability. Specifically, the deposition by activated platelets of the chemokine platelet factor 4 at sites of inflammation promotes adhesion of neutrophils on endothelial cells and thrombogenesis, and it seems deeply involved in the phenomenon of vaccine-induced thrombocytopenia and thrombosis. Importantly, the hyperactivated platelet phenotype along with evidence of cytokine storm, high levels of P-selectin, D-dimer, and, on the other hand, decreased levels of fibrinogen, von Willebrand factor, and thrombocytopenia may be considered suitable biomarkers that distinguish the late stage of COVID-19 progression in critically ill patients.

Platelet binding to polymerizing fibrin is avidity driven and requires activated αIIbβ3 but not fibrin cross-linking

Platelet interaction with polymerizing fibrin is avidity driven and requires activated αIIbβ3 but not fibrin cross-linking.

The mechanism by which αIIbβ3 interacts with polymerizing fibrin differs subtly from the interaction of αIIbβ3 with fibrinogen.

The molecular basis of platelet-fibrin interactions remains poorly understood despite the predominance of fibrin in thrombi. We have studied the interaction of platelets with polymerizing fibrin by adding thrombin to washed platelets in the presence of the peptide RGDW, which inhibits the initial platelet aggregation mediated by fibrinogen binding to αIIbβ3 but leaves intact a delayed increase in light transmission (delayed wave; DW) as platelets interact with the polymerizing fibrin. The DW was absent in platelets from a patient with Glanzmann thrombasthenia, indicating a requirement for αIIbβ3. The DW required αIIbb3 activation and it was inhibited by the αIIbβ3 antagonists eptifibatide and the monoclonal antibody (mAb) 7E3, but only at much higher concentrations than needed to inhibit platelet aggregation initiated by a thrombin receptor activating peptide (T6). Surface plasmon resonance and scanning electron microscopy studies both supported fibrin having greater avidity for αIIbβ3 than fibrinogen rather than greater affinity, consistent with fibrin’s multivalency. mAb 10E5, a potent inhibitor of T6-induced platelet aggregation, did not inhibit the DW, suggesting that fibrin differs from fibrinogen in its mechanism of binding. Inhibition of factor XIII–mediated fibrin cross-linking by >95% reduced the DW by only 32%. Clot retraction showed a pattern of inhibition similar to that of the DW. We conclude that activated αIIbβ3 is the primary mediator of platelet-fibrin interactions leading to clot retraction, and that the interaction is avidity driven, does not require fibrin cross-linking, and is mediated by a mechanism that differs subtly from that of the interaction of αIIbβ3 with fibrinogen.

Structure-function relationship of the platelet glycoprotein VI (GPVI) receptor: does it matter if it is a dimer or monomer?

GPVI is a critical signaling receptor responsible for collagen-induced platelet activation and a promising anti-thrombotic target in conditions such as coronary artery thrombosis, ischemic stroke, and atherothrombosis. This is due to the ability to block GPVI while having minimal effects on hemostasis, making it a more attractive target over current dual-antiplatelet therapy (DAPT) with acetyl salicylic acid and P2Y12 inhibitors where bleeding can be a problem. Our current understanding of how the structure of GPVI relates to function is inadequate and recent studies contradict each other. In this article, we summarize the structure-function relationships underlying the activation of GPVI by its major ligands, including collagen, fibrin(ogen), snake venom toxins and charged exogenous ligands such as diesel exhaust particles. We argue that contrary to popular belief dimerization of GPVI is not required for binding to collagen but serves to facilitate binding through increased avidity, and that GPVI is expressed as a mixture of monomers and dimers on resting platelets, with binding of multivalent ligands inducing higher order clustering.

Platelet transfusion for patients with platelet dysfunction: effectiveness, mechanisms, and unanswered questions

PURPOSE OF REVIEW

In this review, we discuss current clinical guidelines and potential underlying mechanisms regarding platelet transfusion therapy in patients at risk of bleeding, comparing management of patients with thrombocytopenia versus those with qualitative platelet disorders.

RECENT FINDINGS

Platelet transfusion therapy is highly effective in managing bleeding in patients with hypoproliferative thrombocytopenia. Clinical trials have demonstrated that platelet transfusion can be used at a lower trigger threshold and reduced platelet doses, and may be used therapeutically rather than prophylactically in some situations, although additional data are needed. In patients with inherited platelet disorders such as Glanzmann's Thrombasthenia or those with RASGRP2 mutations, platelet transfusion may be ineffective because of competition between transfused and endogenous platelets at the site of vascular injury. Successful management of these patients may require transfusion of additional platelet units, or mechanism-driven combination therapy with other pro-hemostatic agents. In patients on antiplatelet therapy, timing of transfusion and inhibitor mechanism-of-action are key in determining therapeutic success.

SUMMARY

Expanding our understanding of the mechanisms by which transfused platelets exert their pro-hemostatic function in various bleeding disorders will improve the appropriate use of platelet transfusion.

Platelet Proteomes, Pathways, and Phenotypes as Informants of Vascular Wellness and Disease

Platelets rapidly undergo responsive transitions in form and function to repair vascular endothelium and mediate hemostasis. In contrast, heterogeneous platelet subpopulations with a range of primed or refractory phenotypes gradually arise in chronic inflammatory and other conditions in a manner that may indicate or support disease. Qualitatively distinguishable platelet phenotypes are increasingly associated with a variety of physiological and pathological circumstances; however, the origins and significance of platelet phenotypic variation remain unclear and conceptually vague. As changes in platelet function in disease exhibit many similarities to platelets following the activation of platelet agonist receptors, the intracellular responses of platelets common to hemostasis and inflammation may provide insights to the molecular basis of platelet phenotype. Here, we review concepts around how protein-level relations-from platelet receptors through intracellular signaling events-may help to define platelet phenotypes in inflammation, immune responses, aging, and other conditions. We further discuss how representing systems-wide platelet proteomics data profiles as circuit-like networks of causally related intracellular events, or, pathway maps, may inform molecular definitions of platelet phenotype. In addition to offering insights into platelets as druggable targets, maps of causally arranged intracellular relations underlying platelet function can also advance precision and interceptive medicine efforts by leveraging platelets as accessible, dynamic, endogenous, circulating biomarkers of vascular wellness and disease. Graphic Abstract: A graphic abstract is available for this article.

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

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

GPVI (Glycoprotein VI) Interaction With Fibrinogen Is Mediated by Avidity and the Fibrinogen αC-Region

Supplemental Digital Content is available in the text.

Objective:

GPVI (glycoprotein VI) is a key molecular player in collagen-induced platelet signaling and aggregation. Recent evidence indicates that it also plays important role in platelet aggregation and thrombus growth through interaction with fibrin(ogen). However, there are discrepancies in the literature regarding whether the monomeric or dimeric form of GPVI binds to fibrinogen at high affinity. The mechanisms of interaction are also not clear, including which region of fibrinogen is responsible for GPVI binding. We aimed to gain further understanding of the mechanisms of interaction at molecular level and to identify the regions on fibrinogen important for GPVI binding.

Approach and Results:

Using multiple surface- and solution-based protein-protein interaction methods, we observe that dimeric GPVI binds to fibrinogen with much higher affinity and has a slower dissociation rate constant than the monomer due to avidity effects. Moreover, our data show that the highest affinity interaction of GPVI is with the αC-region of fibrinogen. We further show that GPVI interacts with immobilized fibrinogen and fibrin variants at a similar level, including a nonpolymerizing fibrin variant, suggesting that GPVI binding is independent of fibrin polymerization.

Conclusions:

Based on the above findings, we conclude that the higher affinity of dimeric GPVI over the monomer for fibrinogen interaction is achieved by avidity. The αC-region of fibrinogen appears essential for GPVI binding. We propose that fibrin polymerization into fibers during coagulation will cluster GPVI through its αC-region, leading to downstream signaling, further activation of platelets, and potentially stimulating clot growth.

Overcoming challenges in developing small molecule inhibitors for GPVI and CLEC-2

GPVI and CLEC-2 have emerged as promising targets for long-term prevention of both arterial thrombosis and thrombo-inflammation with a decreased bleeding risk relative to current drugs. However, while there are potent blocking antibodies of both receptors, their protein nature comes with decreased bioavailability, making formulation for oral medication challenging. Small molecules are able to overcome these limitations, but there are many challenges in developing antagonists of nanomolar potency, which is necessary when considering the structural features that underlie the interaction of CLEC-2 and GPVI with their protein ligands. In this review, we describe current small-molecule inhibitors for both receptors and strategies to overcome such limitations, including considerations when it comes to in silico drug design and the importance of complex compound library selection.

D-Dimer and Fibrin Degradation Products Impair Platelet Signaling: Plasma D-Dimer Is a Predictor and Mediator of Platelet Dysfunction During Trauma

BACKGROUND

Platelet dysfunction often accompanies trauma-induced coagulopathy. Because soluble fibrin impairs platelet glycoprotein VI (GPVI) signaling and platelets of trauma patients can display impaired calcium mobilization, we explored the role of fibrinolysis on platelet dysfunction during trauma.

METHODS

Convulxin-induced GPVI calcium mobilization was investigated in healthy platelet-rich plasma (PRP) pretreated with thrombin and tissue plasminogen activator (tPA). Blood samples from healthy participants (n = 7) and trauma patients (n = 22) were tested for platelet calcium mobilization, plasma D-dimer, platelet D-dimer binding (via flow cytometry), and platelet lumi-aggregometry.

RESULTS

For healthy platelets, maximal platelet dysfunction was observed when cross-linked soluble fibrin (no tPA) or cross-linked fibrin degradation products (FDPs) were generated in suspension before convulxin stimulation. Lack of fibrin polymerization (inhibited by Gly-Pro-Arg-Pro [GPRP]) or lack of factor XIIIa cross-linking (T101-inhibited) restored GPVI signaling, whereas non-cross-linked FDPs only partially blocked signaling induced by convulxin. In addition, D-dimer added to healthy PRP impaired platelet aggregation and dense granule release induced by various agonists. Plasma D-dimer level was strongly correlated (R = 0.8236) with platelet dysfunction as measured by platelet calcium mobilization induced with various agonists. By 48 to 120 h after trauma, plasma D-dimer levels declined, and platelet function increased significantly but not to healthy levels. Trauma platelets displayed elevated D-dimer binding that was only partially reduced by αIIbβ3-inhibitor GR144053. After 60-minute incubation, washed healthy platelets resuspended in plasma from trauma patients captured approximately 10 000 D-dimer equivalents per platelet.

CONCLUSIONS

During trauma, D-dimer and FDPs inhibit platelets, potentially via GPVI and integrin αIIbβ3 engagement, contributing to a fibrinolysis-dependent platelet loss-of-function phenotype.

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.

The collagen receptor glycoprotein VI promotes platelet-mediated aggregation of β-amyloid

Cerebral amyloid angiopathy (CAA) and β-amyloid (Aβ) deposition in the brain parenchyma are hallmarks of Alzheimer's disease (AD). We previously reported that platelets contribute to Aβ aggregation in cerebral vessels by secreting the factor clusterin upon binding of Aβ40 to the fibrinogen receptor integrin α_IIbβ₃ Here, we investigated the contribution of the collagen receptor GPVI (glycoprotein VI) in platelet-induced amyloid aggregation. Using platelets isolated from GPVI-wild type and GPVI-deficient human donors and mice, we found that Aβ40 bound to GPVI, which induced the release of ATP and fibrinogen, resulting in platelet aggregation. Binding of Aβ40 to integrin α_IIbβ₃, fibrinogen, and GPVI collectively contributed to the formation of amyloid clusters at the platelet surface. Consequently, blockade of α_IIbβ₃ or genetic loss of GPVI reduced amyloid fibril formation in cultured platelets and decreased the adhesion of Aβ-activated platelets to injured carotid arteries in mice. Application of losartan to inhibit collagen binding to GPVI resulted in decreased Aβ40-stimulated platelet activation, factor secretion, and platelet aggregation. Furthermore, the application of GPVI- or integrin-blocking antibodies reduced the formation of platelet-associated amyloid aggregates. Our findings indicate that Aβ40 promotes platelet-mediated amyloid aggregation by binding to both GPVI and integrin α_IIbβ₃ Blocking these pathways may therapeutically reduce amyloid plaque formation in cerebral vessels and the brain parenchyma of patients.

Covid-19: The Rollercoaster of Fibrin(Ogen), D-Dimer, Von Willebrand Factor, P-Selectin and Their Interactions with Endothelial Cells, Platelets and Erythrocytes

Severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2), also known as coronavirus disease 2019 (COVID-19)-induced infection, is strongly associated with various coagulopathies that may result in either bleeding and thrombocytopenia or hypercoagulation and thrombosis. Thrombotic and bleeding or thrombotic pathologies are significant accompaniments to acute respiratory syndrome and lung complications in COVID-19. Thrombotic events and bleeding often occur in subjects with weak constitutions, multiple risk factors and comorbidities. Of particular interest are the various circulating inflammatory coagulation biomarkers involved directly in clotting, with specific focus on fibrin(ogen), D-dimer, P-selectin and von Willebrand Factor (VWF). Central to the activity of these biomarkers are their receptors and signalling pathways on endothelial cells, platelets and erythrocytes. In this review, we discuss vascular implications of COVID-19 and relate this to circulating biomarker, endothelial, erythrocyte and platelet dysfunction. During the progression of the disease, these markers may either be within healthy levels, upregulated or eventually depleted. Most significant is that patients need to be treated early in the disease progression, when high levels of VWF, P-selectin and fibrinogen are present, with normal or slightly increased levels of D-dimer (however, D-dimer levels will rapidly increase as the disease progresses). Progression to VWF and fibrinogen depletion with high D-dimer levels and even higher P-selectin levels, followed by the cytokine storm, will be indicative of a poor prognosis. We conclude by looking at point-of-care devices and methodologies in COVID-19 management and suggest that a personalized medicine approach should be considered in the treatment of patients.

Dominant role of αIIbβ3 in platelet interactions with cross-linked fibrin fragment D-dimer

Although much is known about the interaction of fibrinogen with αIIbβ3, much less is known about the interaction of platelets with cross-linked fibrin. Fibrinogen residue Lys406 plays a vital role in the interaction of fibrinogen with αIIbβ3, but because it participates in fibrin cross-linking, it is not available for interacting with αIIbβ3. We studied the adhesion of platelets and HEK cells expressing normal and constitutively active αIIbβ3 to both immobilized fibrinogen and D-dimer, a proteolytic fragment of cross-linked fibrin, as well as platelet-mediated clot retraction. Nonactivated platelets and HEK cells expressing normal αIIbβ3 adhered to fibrinogen but not D-dimer, whereas activated platelets as well as HEK cells expressing activated αIIbβ3 both bound to D-dimer. Small-molecule antagonists of the αIIbβ3 RGD (Arg-Gly-Asp) binding pocket inhibited adhesion to D-dimer, and an Asp119Ala mutation that disrupts the β3 metal ion-dependent adhesion site inhibited αIIbβ3-mediated adhesion to D-dimer. D-dimer and a polyclonal antibody against D-dimer inhibited clot retraction. The monoclonal antibody (mAb) 10E5, directed at αIIb and a potent inhibitor of platelet interactions with fibrinogen, did not inhibit the interaction of activated platelets with D-dimer or clot retraction, whereas the mAb 7E3, directed at β3, inhibited both phenomena. We conclude that activated, but not nonactivated, αIIbβ3 mediates interactions between platelets and D-dimer, and by extrapolation, to cross-linked fibrin. Although the interaction of αIIbβ3 with D-dimer differs from that with fibrinogen, it probably involves contributions from regions on β3 that are close to, or that are affected by, changes in the RGD binding pocket.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

Progress toward a Glycoprotein VI Modulator for the Treatment of Thrombosis

Pathogenic thrombus formation accounts for the etiology of many serious conditions including myocardial infarction, stroke, deep vein thrombosis, and pulmonary embolism. Despite the development of numerous anticoagulants and antiplatelet agents, the mortality rate associated with these diseases remains high. In recent years, however, significant epidemiological evidence and clinical models have emerged to suggest that modulation of the glycoprotein VI (GPVI) platelet receptor could be harnessed as a novel antiplatelet strategy. As such, many peptidic agents have been described in the past decade, while more recent efforts have focused on the development of small molecule modulators. Herein the rationale for targeting GPVI is summarized and the published GPVI modulators are reviewed, with particular focus on small molecules. A qualitative pharmacophore hypothesis for small molecule ligands at GPVI is also presented.

Fibrin exposure triggers αIIbβ3-independent platelet aggregate formation, ADAM10 activity and glycoprotein VI shedding in a charge-dependent manner

BACKGROUND

Collagen and fibrin engagement and activation of glycoprotein (GP) VI induces proteolytic cleavage of the GPVI ectodomain generating shed soluble GPVI (sGPVI). Collagen-mediated GPVI shedding requires intracellular signalling to release the sGPVI, mediated by A Disintegrin And Metalloproteinase 10 (ADAM10); however, the precise mechanism by which fibrin induces GPVI shedding remains elusive. Plasma sGPVI levels are elevated in patients with coagulopathies, sepsis, or inflammation and can predict onset of sepsis and sepsis-related mortality; therefore, it is clinically important to understand the mechanisms of GPVI shedding under conditions of minimal collagen exposure.

OBJECTIVES

Our aim was to characterize mechanisms by which fibrin-GPVI interactions trigger GPVI shedding.

METHODS

Platelet aggregometry, sGPVI ELISA, and an ADAM10 fluorescence resonance energy transfer assay were used to measure fibrin-mediated platelet responses.

RESULTS

Fibrin induced αIIbβ3-independent washed platelet aggregate formation, GPVI shedding, and increased ADAM10 activity, all of which were insensitive to pre-treatment with inhibitors of Src family kinases but were divalent cation- and metalloproteinase-dependent. In contrast, treatment of washed platelets with other GPVI ligands, collagen, and collagen-related peptide caused αIIbβ3-dependent platelet aggregation and GPVI release but did not increase constitutive ADAM10 activity.

CONCLUSIONS

Fibrin engages GPVI in a manner that differs from other GPVI ligands. Inclusion of polyanionic molecules disrupted fibrin-induced platelet aggregate formation and sGPVI release, suggesting that electrostatic charge may play a role in fibrin/GPVI engagement. It may be feasible to exploit this property and specifically disrupt GPVI/fibrin interactions whilst sparing GPVI/collagen engagement.Fibrin engages GPVI in a manner that differs from other GPVI ligands. Inclusion of polyanionic molecules disrupted fibrin-induced platelet aggregate formation and sGPVI release, suggesting that electrostatic charge may play a role in fibrin/GPVI engagement. It may be feasible to exploit this property and specifically disrupt GPVI/fibrin interactions whilst sparing GPVI/collagen engagement.

Therapeutic strategies for thrombosis: new targets and approaches

Antiplatelet agents and anticoagulants are a mainstay for the prevention and treatment of thrombosis. However, despite advances in antithrombotic therapy, a fundamental challenge is the side effect of bleeding. Improved understanding of the mechanisms of haemostasis and thrombosis has revealed new targets for attenuating thrombosis with the potential for less bleeding, including glycoprotein VI on platelets and factor XIa of the coagulation system. The efficacy and safety of new agents are currently being evaluated in phase III trials. This Review provides an overview of haemostasis and thrombosis, details the current landscape of antithrombotic agents, addresses challenges with preventing thromboembolic events in patients at high risk and describes the emerging therapeutic strategies that may break the inexorable link between antithrombotic therapy and bleeding risk.

Platelets as Potent Signaling Entities in Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a multifactorial disease with a dysregulated circulating inflammatory molecule tendency. T2DM is closely associated with systemic inflammation, endothelial dysfunction, cardiovascular risk, and increased clotting susceptibility. Platelets have fundamental roles in the development and propagation of inflammation and cardiovascular risk. They signal through membrane receptors, resulting in (hyper)activation and release of inflammatory molecules from platelet compartments. This review highlights how circulating inflammatory molecules, acting as platelet receptor ligands, interact with platelets, causing platelets to be potent drivers of systemic inflammation. We conclude by suggesting that focused platelet research in T2DM is an important avenue to pursue to identify novel therapeutic targets, and that platelets could be used as cellular activity sensors themselves.

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.

Inflammatory cytokines in type 2 diabetes mellitus as facilitators of hypercoagulation and abnormal clot formation

BACKGROUND

The global burden of type 2 diabetes mellitus (T2DM), together with the presence of cardiovascular risk in this population, is reaching pandemic levels. A prominent feature of T2DM is chronic and systemic inflammation, with the accompanying presence of circulating and dysregulated inflammatory biomarkers; which in turn is associated with abnormal clot formation.

METHODS

Here, we investigate the correlation between abnormal blood clotting, using thromboelastography (TEG), clot ultrastructure using scanning electron microscopy (SEM) and the presence of a dysregulated inflammatory cytokine profile, by examining various circulating biomarkers.

RESULTS

Our results show that many biomarkers, across TEG, cytokine and lipid groups, were greatly dysregulated in the T2DM sample. Furthermore, our T2DM sample's coagulation profiles were significantly more hypercoagulable when compared to our heathy sample, and ultrastructural analysis confirmed a matted and denser clot structure in the T2DM sample.

CONCLUSIONS

We suggest that dysregulated circulating molecules may in part be responsible for a hypercoagulable state and vascular dysfunction in the T2DM sample. We propose further that a personalized approach could be of great value when planning treatment and tracking the patient health status after embarking on a treatment regimes, and that looking to novel inflammatory and vascular biomarkers might be crucial.

Functional significance of the platelet immune receptors GPVI and CLEC-2

Although platelets are best known for their role in hemostasis, they are also crucial in development, host defense, inflammation, and tissue repair. Many of these roles are regulated by the immune-like receptors glycoprotein VI (GPVI) and C-type lectin receptor 2 (CLEC-2), which signal through an immunoreceptor tyrosine-based activation motif (ITAM). GPVI is activated by collagen in the subendothelial matrix, by fibrin and fibrinogen in the thrombus, and by a remarkable number of other ligands. CLEC-2 is activated by the transmembrane protein podoplanin, which is found outside of the vasculature and is upregulated in development, inflammation, and cancer, but there is also evidence for additional ligands. In this Review, we discuss the physiological and pathological roles of CLEC-2 and GPVI and their potential as targets in thrombosis and thrombo-inflammatory disorders (i.e., disorders in which inflammation plays a critical role in the ensuing thrombosis) relative to current antiplatelet drugs.