Glycoprotein V-Deficient Platelets Have Undiminished Thrombin Responsiveness and Do Not Exhibit a Bernard-Soulier Phenotype

Bernard-Soulier Syndrome: A Review of Epidemiology, Molecular Pathology, Clinical Features, Laboratory Diagnosis, and Therapeutic Management

Bernard-Soulier syndrome (BSS) is an inherited platelet function disorder caused by mutations in the genes that encode the glycoprotein (GP) Ibα and GPIbβ subunits, as well as the GPIX subunit in the GPIbIX complex, which is located on the platelet surface and has roles in platelet adhesion and activation. Patients with autosomal recessively inherited biallelic BSS have a homozygous or compound heterozygous expression in the GPIbα, GPIbβ, and GPIX subunits of the GPIbIX complex. Patients with autosomal dominantly inherited monoallelic BSS have a heterozygous expression in only the GPIbα and GPIbβ subunits of the GPIbIX complex. To date, no BSS mutations in the GP5 gene have been reported. Patients with biallelic form are usually diagnosed at a young age, typically with mucocutaneous bleeding, whereas monoallelic forms are generally identified later in life and are frequently misdiagnosed with immune thrombocytopenic purpura (ITP). In biallelic BSS, giant platelets in the peripheral blood smear, absence of ristocetin-induced platelet aggregation (RIPA) using light transmission aggregometry (LTA), and complete loss of GPIbIX complex in flow cytometry are observed, whereas in monoallelic forms, genetic diagnosis is recommended due to the presence of large platelets in the peripheral blood smear, decreased or normal RIPA response in LTA, and partial loss or normal GPIbIX complex in flow cytometry. Platelet transfusion is the main therapy but recombinant factor VIIa is advised in alloimmunized patients, and allogeneic stem cell transplantation is suggested in refractory cases. Antifibrinolytics and oral contraceptives are utilized as supplementary treatments. Finally, differentiation from ITP is critical due to differences in management. Thus, BSS should be kept in mind in the presence of individuals with chronic persistent thrombocytopenia, positive family history, unresponsive ITP treatment, macrothrombocytopenia, and absence of RIPA response.

Extracorporeal membrane oxygenation aggravates platelet glycoprotein V shedding and δ-granule deficiency in COVID-19-associated acute respiratory distress syndrome

BACKGROUND

Extracorporeal membrane oxygenation (ECMO) is a lifesaving therapy in patients with acute respiratory distress syndrome (ARDS). Hemostatic complications are frequently observed in patients on ECMO and limit the success of this therapy. Platelets are key mediators of hemostasis enabling activation, aggregation, and thrombus formation by coming in contact with exposed matrix proteins via their surface receptors such as glycoprotein (GP) VI or GPIb/V/IX. Recent research has elucidated a regulatory role of the GPV subunit. The cleaved soluble GPV (sGPV) ectodomain was identified to spatiotemporally control fibrin formation through complex formation with thrombin.

OBJECTIVES

We aimed to decipher the impact of ECMO on platelet phenotype and function, including the role of GPV and plasmatic sGPV.

METHODS

We recruited 36 patients with ARDS in the wake of COVID-19 pneumonia and performed a longitudinal comparison of platelet phenotype and function in non-ECMO (n = 23) vs ECMO (n = 13) compared with those of healthy controls. Patients were assessed at up to 3 time points (t1 = days 1-3; t2 = days 4-6; and t3 = days 7-14 after cannulation/study inclusion).

RESULTS

Agonist-induced platelet activation was assessed by flow cytometry and revealed decreased GPIIb/IIIa activation and α-granule release in all ARDS patients. During ECMO treatment, agonist-induced δ-granule release continuously decreased, which was independently confirmed by electron microscopy and was associated with a prolonged in vitro bleeding time. GPV expression on the platelet surface markedly decreased in ECMO patients compared with that in non-ECMO patients. Plasma sGPV levels were increased in ECMO patients and were associated with poor outcome.

CONCLUSION

Our data demonstrate an ECMO-intrinsic platelet δ-granule deficiency and hemostatic dysfunction beyond the underlying ARDS.

Platelet glycoprotein V spatio-temporally controls fibrin formation

The activation of platelets and coagulation at vascular injury sites is crucial for haemostasis but can promote thrombosis and inflammation in vascular pathologies. Here, we delineate an unexpected spatio-temporal control mechanism of thrombin activity that is platelet orchestrated and locally limits excessive fibrin formation after initial haemostatic platelet deposition. During platelet activation, the abundant platelet glycoprotein (GP) V is cleaved by thrombin. We demonstrate with genetic and pharmacological approaches that thrombin-mediated shedding of GPV does not primarily regulate platelet activation in thrombus formation, but rather has a distinct function after platelet deposition and specifically limits thrombin-dependent generation of fibrin, a crucial mediator of vascular thrombo-inflammation. Genetic or pharmacologic defects in haemostatic platelet function are unexpectedly attenuated by specific blockade of GPV shedding, indicating that the spatio-temporal control of thrombin-dependent fibrin generation also represents a potential therapeutic target to improve haemostasis.

Platelet findings in 22q11.2 deletion syndrome correlate with disease manifestations but do not correlate with GPIb surface expression

Prior studies have demonstrated that patients with chromosome 22q11.2 deletion syndrome (22q11.2DS) have lower platelet counts (PC) compared to non-deleted populations. They also have an increased mean platelet volume. The mechanism for this has been postulated to be haploinsufficiency of the GPIBB gene. We examined platelet parameters, deletion size and factors known to influence counts, including status of thyroid hormone and congenital heart disease (CHD), in a population of 825 patients with 22q11.2DS. We also measured surface expression of GPIB-IX complex by flow cytometry. The major determinant of PC was deletion status of GP1BB, regardless of surface expression or other factors. Patients with nested distal chromosome 22q11.2 deletions (those with GP1BB present) had higher PCs than those with proximal deletions where GP1BB is deleted. Patients with 22q11.2DS also demonstrated an accelerated PC decrease with age, occurring in childhood. These data demonstrate that genes within the proximal deletion segment drive PC differences in 22q11.2DS and suggest that PC reference ranges may need to be adjusted for age and deletion size in 22q11.2DS populations. Bleeding did not correlate with either platelet count or GPIb expression. Further studies into drivers of expression of GPIb and associations with severe thrombocytopenia and immune thrombocytopenia are needed to inform clinical care.

Glycoprotein V : the unsolved GPV puzzle

Glycoprotein V (GPV) is a highly expressed 82 KDa platelet surface transmembrane protein which is loosely attached to the GPIb-IX complex. Despite remaining questions concerning its function, GPV presents several unique features which have repercussions in hematology, atherothrombosis, immunology and transfusion. GPV is specifically expressed in platelets and megakaryocytes and is an ideal marker and reporter gene for the late stages of megakaryopoiesis. The ectodomain of GPV can be released by a number of proteases, namely thrombin, elastase and ADAM10 and 17. Although it was originally proposed as a thrombin receptor, this hypothesis was abandoned since thrombin activation was preserved after blockade of GPV cleavage and in Gp5 knockout mice. The combined potential of GPV to reflect the direct action of thrombin, platelet exposure to strong agonists and inflammatory conditions has led one to evaluate its utility as a marker in the context of atherothrombosis. Increased plasma levels of soluble GPV have notably been recorded in myocardial infarction, stroke and venous thromboembolism. It is also highly valued in transfusion to monitor platelet storage lesions. GPV presents several polymorphisms, which are a possible source of alloantibodies, while autoantibodies have been frequently detected in immune thrombocytopenia. The real biological function of this glycoprotein nevertheless remains an enigma, despite the respectively decreased and increased responses to low concentrations of collagen and thrombin observed in Gp5 knockout mice. Current studies are exploring its role in modulating general or VWF-induced platelet signaling, which could bear relevance in thrombosis and platelet clearance.

Thrombocytopathies: Not Just Aggregation Defects-The Clinical Relevance of Procoagulant Platelets

Platelets are active key players in haemostasis. Qualitative platelet dysfunctions result in thrombocytopathies variously characterized by defects of their adhesive and procoagulant activation endpoints. In this review, we summarize the traditional platelet defects in adhesion, secretion, and aggregation. In addition, we review the current knowledge about procoagulant platelets, focusing on their role in bleeding or thrombotic pathologies and their pharmaceutical modulation. Procoagulant activity is an important feature of platelet activation, which should be specifically evaluated during the investigation of a suspected thrombocytopathy.

The impact of GPIbα on platelet-targeted FVIII gene therapy in hemophilia A mice with pre-existing anti-FVIII immunity

Essentials Platelet-specific FVIII gene therapy is effective in hemophilia A mice even with inhibitors. The impact of platelet adherence via VWF/GPIbα binding on platelet gene therapy was investigated. GPIbα does not significantly affect platelet gene therapy of hemophilia A with inhibitors. Platelet gene therapy induces immune tolerance in hemophilia A mice with pre-existing immunity. SUMMARY: Background We have previously demonstrated that von Willebrand factor (VWF) is essential in platelet-specific FVIII (2bF8) gene therapy of hemophilia A (HA) with inhibitory antibodies (inhibitors). At the site of injury, platelet adherence is initiated by VWF binding to the platelet GPIb complex. Objective To investigate the impact of GPIbα on platelet gene therapy of HA with inhibitors. Methods Platelet-FVIII expression was introduced by 2bF8 lentivirus (2bF8LV) transduction of hematopoietic stem cells (HSCs) from GPIbαnull (Ibnull ) mice or rhF8-primed FVIIInull (F8null ) mice followed by transplantation into lethally irradiated rhF8-primed F8null recipients. Animals were analyzed by flow cytometry, FVIII assays and the tail bleeding test. Results After transplantation, 99% of platelets were derived from donors. The macrothrombocytopenia phenotype was maintained in F8null mice that received 2bF8LV-transduced Ibnull HSCs (2bF8-Ibnull /F8null ). The platelet-FVIII expression level in 2bF8-Ibnull /F8null recipients was similar to that obtained from F8null mice that received 2bF8LV-transduced F8null HSCs (2bF8-F8null /F8null ). The tail bleeding test showed that the remaining hemoglobin level in the 2bF8-Ibnull /F8null group was significantly higher than in the F8null control group, but there was no significant difference between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The half-life of inhibitor disappearance time was comparable between the 2bF8-Ibnull /F8null and 2bF8-F8null /F8null groups. The rhF8 re-challenge did not elicit a memory immune response once inhibitor titers dropped to undetectable levels after 2bF8 gene therapy. Conclusion GPIbα does not significantly impact platelet gene therapy of HA with inhibitors. 2bF8 gene therapy restores hemostasis and promotes immune tolerance in HA mice with pre-existing immunity.

14-3-3 proteins in platelet biology and glycoprotein Ib-IX signaling

Members of the 14-3-3 family of proteins function as adapters/modulators that recognize phosphoserine/phosphothreonine-based binding motifs in many intracellular proteins and play fundamental roles in signal transduction pathways of eukaryotic cells. In platelets, 14-3-3 plays a wide range of regulatory roles in phosphorylation-dependent signaling pathways, including G-protein signaling, cAMP signaling, agonist-induced phosphatidylserine exposure, and regulation of mitochondrial function. In particular, 14-3-3 interacts with several phosphoserine-dependent binding sites in the major platelet adhesion receptor, the glycoprotein Ib-IX complex (GPIb-IX), regulating its interaction with von Willebrand factor (VWF) and mediating VWF/GPIb-IX-dependent mechanosignal transduction, leading to platelet activation. The interaction of 14-3-3 with GPIb-IX also plays a critical role in enabling the platelet response to low concentrations of thrombin through cooperative signaling mediated by protease-activated receptors and GPIb-IX. The various functions of 14-3-3 in platelets suggest that it is a possible target for the treatment of thrombosis and inflammation.

Epac1-deficient mice have bleeding phenotype and thrombocytes with decreased GPIbβ expression

Epac1 (Exchange protein directly activated by cAMP 1) limits fluid loss from the circulation by tightening the endothelial barrier. We show here that Epac1-/- mice, but not Epac2-/- mice, have prolonged bleeding time, suggesting that Epac1 may limit fluid loss also by restraining bleeding. The Epac1-/- mice had deficient in vitro secondary hemostasis. Quantitative comprehensive proteomics analysis revealed that Epac1-/- mouse platelets (thrombocytes) had unbalanced expression of key components of the glycoprotein Ib-IX-V (GPIb-IX-V) complex, with decrease of GP1bβ and no change of GP1bα. This complex is critical for platelet adhesion under arterial shear conditions. Furthermore, Epac1-/- mice have reduced levels of plasma coagulation factors and fibrinogen, increased size of circulating platelets, increased megakaryocytes (the GP1bβ level was decreased also in Epac1-/- bone marrow) and higher abundance of reticulated platelets. Viscoelastic measurement of clotting function revealed Epac1-/- mice with a dysfunction in the clotting process, which corresponds to reduced plasma levels of coagulation factors like factor XIII and fibrinogen. We propose that the observed platelet phenotype is due to deficient Epac1 activity during megakaryopoiesis and thrombopoiesis, and that the defects in blood clotting for Epac1-/- is connected to secondary hemostasis.

Platelet clearance via shear-induced unfolding of a membrane mechanoreceptor

Mechanisms by which blood cells sense shear stress are poorly characterized. In platelets, glycoprotein (GP)Ib–IX receptor complex has been long suggested to be a shear sensor and receptor. Recently, a relatively unstable and mechanosensitive domain in the GPIbα subunit of GPIb–IX was identified. Here we show that binding of its ligand, von Willebrand factor, under physiological shear stress induces unfolding of this mechanosensory domain (MSD) on the platelet surface. The unfolded MSD, particularly the juxtamembrane ‘Trigger' sequence therein, leads to intracellular signalling and rapid platelet clearance. These results illustrate the initial molecular event underlying platelet shear sensing and provide a mechanism linking GPIb–IX to platelet clearance. Our results have implications on the mechanism of platelet activation, and on the pathophysiology of von Willebrand disease and related thrombocytopenic disorders. The mechanosensation via receptor unfolding may be applicable for many other cell adhesion receptors.

The platelets detect and respond to shear stress generated by blood flow. Here the authors show that the binding of the soluble von Willebrand factor to its receptor GPIba under physiological shear stress induces receptor's domain unfolding on the platelet and signalling into the platelet, leading to platelets clearance.

First molecular cloning and gene expression analysis of teleost CD42 (glycoprotein Ib beta chain) GPIb-IX-V subunit from rock bream, Oplegnathus fasciatus

CD42 is a platelet membrane glycoprotein Ib that plays a key role in haemostasis and thrombin-induced platelet activation. Here, we report the molecular cloning and sequence analysis of the CD42c gene from rock bream (Oplegnathus fasciatus). Rock bream CD42 (RbCD42c) gene expression profiles were determined after infection with Streptococcus iniae, Edwardsiella tarda and red seabream iridovirus (RSIV). The full-length RbCD42c cDNA contained an open reading frame of 624 bp encoding 207 amino acids. The deduced amino acid sequences of the leucine-rich repeat (LRR)-N terminal and LRR-C terminal were conserved between fish and mammals. RbCD42c was highly expressed in red blood cells, spleen, gill, liver and kidney of healthy rock bream. The RbCD42c gene was not significantly up- or downregulated after E. tarda exposure. However, RbCD42c gene expression was upregulated in kidney, spleen and gill after S. iniae infection. RbCD42c was upregulated in spleen, liver and gill, but downregulated in kidney 24 and 48 h after RSIV infection. These results suggest that RbCD42c has different expression patterns after infection with bacterial or viral pathogens. This gene may be directly involved in haemostasis.

Of von Willebrand factor and platelets

Hemostasis and pathological thrombus formation are dynamic processes that require multiple adhesive receptor-ligand interactions, with blood platelets at the heart of such events. Many studies have contributed to shed light on the importance of von Willebrand factor (VWF) interaction with its platelet receptors, glycoprotein (GP) Ib-IX-V and αIIbβ3 integrin, in promoting primary platelet adhesion and aggregation following vessel injury. This review will recapitulate our current knowledge on the subject from the rheological aspect to the spatio-temporal development of thrombus formation. We will also discuss the signaling events generated by VWF/GPIb-IX-V interaction, leading to platelet activation. Additionally, we will review the growing body of evidence gathered from the recent development of pathological mouse models suggesting that VWF binding to GPIb-IX-V is a promising target in arterial and venous pathological thrombosis. Finally, the pathological aspects of VWF and its impact on platelets will be addressed.

Genetic deletion of platelet glycoprotein Ib alpha but not its extracellular domain protects from atherosclerosis

The pathogenesis of atherosclerosis involves the interplay of haematopoietic, stromal and endothelial cells. Platelet interactions with endothelium and leukocytes are pivotal for atherosclerosis promotion. Glycoprotein (GP) Ibα is the ligand-binding subunit of the platelet GPIb-IX-V receptor complex; its deficiency causes the Bernard-Soulier syndrome (BSS), characterised by absent platelet GPIb-IX-V, macrothrombocytopenia and bleeding. We designed this study to determine the role of platelet GPIbα in the pathogenesis of atherosclerosis using two unique knockout models. Ldlr-/- mice were reconstituted with wild-type (wt), GPIbα-/- (lacks GPIbα) or chimeric IL-4R/GPIbα-Tg (lacks GPIbα extracellular domain) bone marrow and assayed for atherosclerosis development after feeding with pro-atherogenic "western diet". Here, we report that Ldlr-/-mice reconstituted with GPIbα-/- bone marrow developed less atherosclerosis compared to wt controls; accompanied by augmented accumulation of pro-inflammatory CD11b+ and CD11c+ myeloid cells, reduced oxLDL uptake and decreased TNF and IL 12p35 gene expression in the aortas. Flow cytometry and live cell imaging in whole blood-perfused microfluidic chambers revealed reduced platelet-monocyte aggregates in GPIbα-/- mice, which resulted in decreased monocyte activation. Interestingly, Ldlr-/-mice reconstituted with IL-4R/GPIbα-Tg bone marrow, producing less abnormal platelets, showed atherosclerotic lesions similar to wt mice. Platelet interaction with blood monocytes and accumulation of myeloid cells in the aortas were also essentially unaltered. Moreover, only complete GPIbα ablation altered platelet microparticles and CCL5 chemokine production. Thus, atherosclerosis reduction in mice lacking GPIbα may not result from the defective GPIbα-ligand binding, but more likely is a consequence of functional defects of GPIbα-/- platelets and reduced blood platelet counts.

The organizing principle of the platelet glycoprotein Ib-IX-V complex

The glycoprotein (GP)Ib-IX-V complex is the platelet receptor for von Willebrand factor and many other molecules that are critically involved in hemostasis and thrombosis. The lack of functional GPIb-IX-V complexes on the platelet surface is the cause of Bernard-Soulier syndrome, a rare hereditary bleeding disorder that is also associated with macrothrombocytopenia. GPIb-IX-V contains GPIbα, GPIbβ, GPIX and GPV subunits, all of which are type I transmembrane proteins containing leucine-rich repeat domains. Although all of the subunits were identified decades ago, not until recently did the mechanism of complex assembly begin to emerge from a systematic characterization of inter-subunit interactions. This review summarizes the forces driving the assembly of GPIb-IX-V, discusses their implications for the pathogenesis of Bernard-Soulier syndrome, and identifies questions that remain about the structure and organization of GPIb-IX-V.

Transmembrane domains are critical to the interaction between platelet glycoprotein V and glycoprotein Ib-IX complex

BACKGROUND

The glycoprotein (GP) Ib-IX-V complex, the von Willebrand factor receptor on the platelet surface, is critically involved in hemostasis and thrombosis. The GPV subunit interacts with GPIb-IX to form the GPIb-IX-V complex, but the underlying molecular basis remains unclear. It was observed earlier that efficient expression of GPV in the plasma membrane requires co-expression of GPIb-IX.

OBJECTIVES AND METHODS

Hypothesizing that GPIb-IX stabilizes GPV through direct interaction and consequently enhances GPV surface expression, we aim in this study to identify structural elements in the complex that mediate the interaction between GPV and GPIb-IX by analyzing mutational effects on GPV surface expression in transfected Chinese hamster ovary cells.

RESULTS

Enhancement of GPV surface expression by GPIb-IX requires transmembrane domains of both GPV and GPIbα, as replacing the GPV transmembrane domain with an unrelated poly-leucine-alanine sequence abolished the enhancing effect of GPIb-IX. Additional mutagenesis analysis of the GPV transmembrane helix identified three helical sides containing conserved polar residues as critical to efficient GPV surface expression. Similarly, replacing residues in three sides (Gly495/Ala502/Leu509, Phe491/Trp498/Val505, and Y492/L499/L506) of the GPIbα transmembrane domain with leucines preserved the surface expression level of GPIb-IX but significantly altered that of GPV.

CONCLUSIONS

Our results demonstrate for the first time the importance of transmembrane domains for efficient surface expression of GPV and suggest that GPV and GPIbα transmembrane domains interact with each other, contributing to assembly of the GPIb-IX-V complex.

Quaternary organization of GPIb-IX complex and insights into Bernard-Soulier syndrome revealed by the structures of GPIbβ and a GPIbβ/GPIX chimera

Platelet GPIb-IX receptor complex has 3 subunits GPIbα, GPIbβ, and GPIX, which assemble with a ratio of 1:2:1. Dysfunction in surface expression of the complex leads to Bernard-Soulier syndrome. We have crystallized the GPIbβ ectodomain (GPIbβ(E)) and determined the structure to show a single leucine-rich repeat with N- and C-terminal disulphide-bonded capping regions. The structure of a chimera of GPIbβ(E) and 3 loops (a,b,c) taken from the GPIX ectodomain sequence was also determined. The chimera (GPIbβ(Eabc)), but not GPIbβ(E), forms a tetramer in the crystal, showing a quaternary interface between GPIbβ and GPIX. Central to this interface is residue Tyr106 from GPIbβ, which inserts into a pocket generated by 2 loops (b,c) from GPIX. Mutagenesis studies confirmed this interface as a valid representation of interactions between GPIbβ and GPIX in the full-length complex. Eight GPIbβ missense mutations identified from patients with Bernard-Soulier syndrome were examined for changes to GPIb-IX complex surface expression. Two mutations, A108P and P74R, were found to maintain normal secretion/folding of GPIbβ(E) but were unable to support GPIX surface expression. The close structural proximity of these mutations to Tyr106 and the GPIbβ(E) interface with GPIX indicates they disrupt the quaternary organization of the GPIb-IX complex.

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.

Heat-shock protein gp96/grp94 is an essential chaperone for the platelet glycoprotein Ib-IX-V complex

The platelet glycoprotein Ib-IX-V complex (GPIb-IX-IV) is the receptor for VWF and is responsible for VWF-mediated platelet activation and aggregation. Loss of the GPIb-IX-V complex is pathogenic for Bernard-soulier Syndrome (BSS), which is characterized by macrothrombocytopenia and impaired platelet function. It remains unclear how the GPIb-IX-V complex is assembled and whether there is a role for a specific molecular chaperone in the process. In the present study, we report that the assembly of the GPIb-IX-V complex depends critically on a molecular chaperone in the endoplasmic reticulum (ER): gp96 (also known as grp94 and HSP90b1). gp96/grp94 deletion in the murine hematopoietic system results in thrombocytopenia, prolonged bleeding time, and giant platelets that are clinically indistinguishable from human BSS. Loss of gp96/grp94 in vivo and in vitro leads to the concomitant reduction in GPIb-IX complex expression due to ER-associated degradation. We further demonstrate that gp96/grp94 binds selectively to the GPIX subunit, but not to gpIbα or gpIbβ. Therefore, we identify the platelet GPIX subunit of the GPIb-IX-V complex as an obligate and novel client of gp96/grp94.

A central role of GPIb-IX in the procoagulant function of platelets that is independent of the 45-kDa GPIbα N-terminal extracellular domain

Activated platelets become procoagulant and efficiently promote the conversion of prothrombin to thrombin. A role of the GPIb-V-IX complex has long been postulated in view of the decreased prothrombin consumption in Bernard-Soulier patients. We evaluated the impact of GPIb-V-IX deficiency and the requirement for the GPIbα extracellular domain. In GPIbβ−/− mice, thrombin generation was profoundly decreased in tissue factor– or collagen-related peptide (CRP)–activated platelet-rich plasma and in washed platelets supplemented with normal plasma or with FVa, FXa, and prothrombin. Phosphatidylserine (PS) exposure was similarly decreased in response to thrombin, CRP, or CRP + PAR4 peptide despite a normal platelet phospholipid composition. The hypothesis that these defects originate from lack of the GPIbα N-terminal domain was evaluated after its removal from normal mouse and human platelets with Nk protease or O-sialoglycoprotein endopeptidase. Unexpectedly, the treated platelets exhibited normal thrombin generation and PS exposure, indicating that GPIb-V-IX regulates procoagulant activity independently of its GPIbα-binding region. These results suggested a more general structuring role through intracellular cytoskeleton-anchoring portions regulating responses leading to PS exposure. This hypothesis was supported by the decreased calcium mobilization observed in GPIbβ−/− platelets in response to several agonists, some acting independently of GPIb, in contrast to the normal calcium responses in Nk protease–treated platelets.

Specific heteromeric association of four transmembrane peptides derived from platelet glycoprotein Ib-IX complex

As the receptor on the platelet surface for von Willebrand factor, glycoprotein (GP) Ib-IX complex is critically involved in hemostasis and thrombosis. How the complex is assembled from GP Ibalpha, GP Ib beta and GP IX subunits, all of which are type I transmembrane proteins, is not entirely clear. Genetic and mutational analyses have identified the transmembrane (TM) domains of these subunits as active participants in assembly of the complex. In this study, peptides containing the transmembrane domain of each subunit have been produced and their interaction with one another characterized. Only the Ib beta TM sequence, but not the Ibalpha and IX counterparts, can form homo-oligomers in SDS-PAGE and TOXCAT assays. Following up on our earlier observation that a Ib beta-Ibalpha-Ib beta peptide complex (alphabeta(2)) linked through native juxtamembrane disulfide bonds could be produced from isolated Ibalpha and Ib beta TM peptides in detergent micelles, we show here that addition of the IX TM peptide facilitates formation of the native alphabeta(2) complex, reproducing the same effect by the IX subunit in cells expressing the GP Ib-IX complex. Specific fluorescence resonance energy transfer was observed between donor-labeled alphabeta(2) peptide complex and acceptor-conjugated IX TM peptide in micelles. Finally, the mutation D135K in the IX TM peptide could hamper both the formation of the alphabeta(2) complex and the energy transfer, consistent with its reported effect in the full-length complex. Overall, our results have demonstrated directly the native-like heteromeric interaction among the isolated Ibalpha, Ib beta and IX TM peptides, which provides support for the four-helix bundle model of the TM domains in the GP Ib-IX complex and paves the way for further structural analysis. The methods developed in this study may be applicable to other studies of heteromeric interaction among multiple TM helices.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

In vivo thrombus formation in murine models

Platelets play a central role in hemostasis, but also in atherothrombosis, as they rapidly adhere to tissue and to one another as a response to any vascular injury. This process involves a large number of surface receptors, signaling pathways, and enzymatic cascades as well as their complex interplay. Although in vitro experiments proved successful in both identifying new receptors and pathways and developing potent and selective antithrombotic drugs, in vitro research cannot mimic the myriad hemodynamic and spatiotemporal cellular and molecular interactions that occur during the generation and propagation of thrombi in vivo. Animal models, and, with the availability of genetically modified mouse strains and of modern intravital imaging techniques, mouse models in particular, have opened new ways to identify both individual roles and the interplay of platelet proteins in complex in vivo settings. In vivo models revealed the important role of, eg, Gas6 or blood coagulation factor XII in thrombus formation, and results obtained in in vivo models raised the interesting possibility that (physiologic) hemostasis and (pathologic) thrombosis might represent 2 mechanistically different processes. This review summarizes in vivo findings that contributed significantly to our understanding of hemostatic and thrombotic processes and which may help to guide future research.

A guide to murine platelet structure, function, assays, and genetic alterations

Platelets play an important role in hemostasis, thrombosis and several other biological processes. The adaptability of mice to genetic manipulation and their genetic similarity to humans has resulted in a plethora of murine models to study platelet function. Although murine platelets differ from human platelets with regard to size, number and structure, functionally they are very similar. Thus, studies which employed these model systems have greatly improved our current understanding of the contribution of platelets to hemostasis and thrombosis. This review presents general recommendations with respect to collection, isolation and processing of murine platelets. It also describes the assays currently available to study platelet function and critically assesses their utility. The extensive literature on the effects of genetic alterations on murine platelet function is considered in detail. This review is intended to provide a convenient source of reference for platelet investigators.

Inhibition of adhesive and signaling functions of the platelet GPIb-V-IX complex by a cell penetrating GPIbalpha peptide

BACKGROUND

Interaction between the platelet glycoprotein (GP)Ib-V-IX complex and von Willebrand factor (VWF) is critical for initiating platelet-vessel wall contacts, particularly under high shear conditions. This interaction also plays an important role in initiating platelet activation through the generation of intracellular signals resulting in platelet shape change and integrin alpha(IIb)beta3 activation.

OBJECTIVE

A cell-penetrating peptide strategy was used to study the role of the intracellular domain of the GPIbalpha subunit in VWF/GPIb-V-IX-dependent adhesion and activation.

METHODS

Peptides of 11-13 amino acids, covering the 557-610 region, were coupled to a nine-arginine permeating tag (R9) and the effects of their cell entry on VWF-dependent responses were analyzed.

RESULTS

The R9alpha557 peptide corresponding to the 557-569 segment reduced platelet agglutination in response to VWF, while the other peptides had no effect. The decreased platelet agglutination appeared to be an indirect consequence of adenosine diphosphate release as a normal response was restored by apyrase or a P2Y1 receptor antagonist. A more direct effect of R9alpha557 on GPIb VWF-dependent functions was observed in adhesion studies on a VWF matrix, where it decreased platelet adhesion and profoundly inhibited filopodia formation. In addition, cell adhesion was reduced and shape change absent when Chinese hamster ovary cells expressing the GPIb-IX complex were incubated with R9alpha557.

CONCLUSION

This study performed in intact platelets suggests a functional role of the 557-569 domain of GPIbalpha in controlling VWF-dependent adhesion and signaling.

Platelets in atherothrombosis: lessons from mouse models

Platelets play a central role in hemostasis and thrombosis but also in the initiation of atherosclerosis, making platelet receptors and their intracellular signaling pathways important molecular targets for antithrombotic and anti-inflammatory therapy. Historically, much of the knowledge about hemostasis and thrombosis has been derived from patients suffering from bleeding and thrombotic disorders and the identification of the underlying molecular defects. In recent years, the availability of genetically modified mouse strains with defined defects in platelet function and the development of in vivo models to assess platelet-related physiologic and pathophysiologic processes have opened new ways to identify the individual roles and the interplay of platelet proteins in adhesion, activation, aggregation, secretion, and procoagulant activity in vitro and in vivo. This review will summarize key findings made by these approaches and discuss them in the context of human disease.

Platelet adhesion signalling and the regulation of thrombus formation

Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin alpha(2)beta(1). These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin alpha(IIb)beta(3), a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins alpha(2)beta(1) and alpha(IIb)beta(3) contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.

Evidence for a role of ADAM17 (TACE) in the regulation of platelet glycoprotein V

Glycoprotein V (GPV) is a subunit of the GPIb-IX-V receptor for von Willebrand factor and thrombin and has been shown to modulate platelet responses to the two strongest physiological agonists, thrombin and collagen. Thrombin directly cleaves GPV from the platelet surface, yielding a 69-kDa fragment GPV f1 of unknown function. We show here that a approximately 82-kDa fragment of GPV is shed from the platelet surface upon cellular activation with phorbol 12-myristate 13-acetate or the collagen-related peptide. This shedding was inhibited by the broad range metalloproteinase inhibitor GM6001, the two potent ADAM17 inhibitors GW280264X and TAPI-2, and was absent in mice lacking functional ADAM17 (ADAM17 lacking Zn-binding domain; ADAM17(DeltaZn/DeltaZn)). Furthermore, we show that recombinant ADAM17 ectodomain efficiently releases GPV from the platelet surface. GPV is known to be associated with the intracellular regulatory protein calmodulin, which has previously been shown to be involved in ADAM17-mediated shedding of l-selectin from the surface of leukocytes. As in these reports, inhibition of calmodulin led to rapid GPV shedding from the platelet surface, a process that was again blocked by GM6001 or ADAM17 inhibitors and that was absent in ADAM17(DeltaZn/DeltaZn) mice. Inhibition of outside-in signaling through GPIIb/IIIa did not significantly affect GPV shedding, excluding an essential role of this pathway for the regulation of ADAM17 activity. These results demonstrate that GPV is cleaved upon agonist-induced platelet activation and show that ADAM17 is the major enzyme mediating this process.

The snake venom protein botrocetin acts as a biological brace to promote dysfunctional platelet aggregation

Botrocetin is a snake venom protein that enhances the affinity of the A1 domain of plasma von Willebrand factor (vWF) for the platelet receptor glycoprotein Ibalpha (GPIbalpha), an event that contributes to bleeding and host death. Here we describe a kinetic and crystallographic analysis of this interaction that reveals a novel mechanism of affinity enhancement. Using high-temporal-resolution microscopy, we show that botrocetin decreases the GPIbalpha off-rate two-fold in both human and mouse complexes without affecting the on-rate. The key to this behavior is that, upon binding of GPIbalpha to vWF-A1, botrocetin prebound to vWF-A1 makes no contacts initially with GPIbalpha, but subsequently slides around the A1 surface to form a new interface. This two-step mechanism and flexible coupling may prevent adverse alterations in on-rate of GPIbalpha for vWF-A1, and permit adaptation to structural differences in GPIbalpha and vWF in several prey species.

Biosynthesis of platelet glycoprotein V expressed as a single subunit or in association with GPIb-IX

Glycoprotein (GP) V is noncovalently linked to GPIbalpha, GPIbbeta and GPIX within the platelet GPIb-V-IX complex, a receptor for von Willebrand factor and thrombin. Two functions have been ascribed to GPV, namely, the modulation of thrombin- and collagen-dependent platelet responses. The biosynthesis of this molecule was investigated in pulse-chase metabolic labelling experiments performed in CHO cell lines transfected with GPV, alone or in the presence of GPIb-IX. GPV could not be detected at the surface of cells expressing the single subunit but was found instead as a soluble form in the culture medium. In pulse-chase studies, an immature 70 kDa protein was detected in cell lysates, whereas a fully processed 80-82 kDa form was only observed in the culture supernatants at later chase times. Immature GPV was N-glycosylated and retained before the medial Golgi while the secreted molecule contained complex sialylated sugars. The mature soluble form of GPV was produced by an enzymatic cleavage which was not affected by inhibitors of proteasome, calpain or metalloproteinases. When GPV was cotransfected with GPIb-IX, the former was no longer found in the culture supernatant but was retained in the cell membrane as shown by fluorescence-activated cell sorting and confocal microscopy analyses. Surface expressed GPV was processed from an immature 70 kDa form to produce a mature 80 kDa protein, processing similar to the intracellular trafficking of GPIbalpha. These results indicate that correct biosynthesis and surface expression of GPV in platelets requires the presence of the other subunits of the GPIb-V-IX complex.

Genetic deletion of mouse platelet glycoprotein Ibbeta produces a Bernard-Soulier phenotype with increased alpha-granule size

Here we report the characterization of a mouse model of the Bernard-Soulier syndrome generated by a targeted disruption of the gene encoding the glycoprotein (GP) Ibbeta subunit of the GP Ib-IX complex. Similar to a Bernard-Soulier model generated by disruption of the mouse GP Ibalpha subunit, GP Ibbeta(Null) mice display macrothrombocytopenia and a severe bleeding phenotype. When examined by transmission electron microscopy, the large platelets produced by a GP Ibbeta(Null) genotype revealed alpha-granules with increased size as compared with the alpha-granules from control mouse platelets. Data are presented linking the overexpression of a septin protein, SEPT5, to the presence of larger alpha-granules in the GP Ibbeta(Null) platelet. The SEPT5 gene resides approximately 250 nucleotides 5' to the GP Ibbeta gene and has been associated with modulating exocytosis from neurons and platelets as part of a presynaptic protein complex. Fusion mRNA transcripts present in megakaryocytes can contain both the SEPT5 and GP Ibbeta coding sequences as a result in an imperfect polyadenylation signal within the 3' end of both the human and mouse SEPT5 genes. We observed a 2- to 3-fold increase in SEPT5 protein levels in platelets from GP Ibbeta(Null) mice. These results implicate SEPT5 levels in the maintenance of normal alpha-granule size and may explain the variant granules associated with human GP Ibbeta mutations and the Bernard-Soulier syndrome.

Antiplatelet therapy: in search of the 'magic bullet'

The central importance of platelets in the development of arterial thrombosis and cardiovascular disease is well established. No other single cell type is responsible for as much morbidity and mortality as the platelet and, as a consequence, it represents a major target for therapeutic intervention. The growing awareness of the importance of platelets is reflected in the increasing number of patients receiving antiplatelet therapy, a trend that is likely to continue in the future. There are, however, significant drawbacks with existing therapies, including issues related to limited efficacy and safety. The discovery of a 'magic bullet' that selectively targets pathological thrombus formation without undermining haemostasis remains elusive, although recent progress in unravelling the molecular events regulating thrombosis has provided promising new avenues to solve this long-standing problem.

Glycoprotein Ib-IX-V

Glycoprotein (GP) Ib-IX-V is a remarkable platelet adhesion receptor of the leucine-rich repeat family. It has evolved to fulfil its major function of initiating platelet aggregation (thrombus formation) at high-shear stress in flowing blood. In addition to binding von Willebrand factor (vWF) in subendothelial matrix or plasma to trigger platelet aggregation, GPIb-IX-V also binds counter-receptors, alphaMbeta2 (Mac-1) on neutrophils or P-selectin on activated platelets or endothelial cells. GPIb-IX-V ligands also include alpha-thrombin, clotting factors XI/XIIa, and high-molecular-weight kininogen. Interactions involving GPIb-IX-V are therefore central to vascular processes of thrombosis and inflammation, and the receptor is under intense scrutiny as a potential therapeutic target.

Platelet-collagen interaction: is GPVI the central receptor?

At sites of vascular injury, platelets come into contact with subendothelial collagen, which triggers their activation and the formation of a hemostatic plug. Besides glycoprotein Ib (GPIb) and alphaIIbbeta3 integrin, which indirectly interact with collagen via von Willebrand factor (VWF), several collagen receptors have been identified on platelets, most notably alpha2beta1 integrin and the immunoglobulin (Ig) superfamily member GPVI. Within the last few years, major advances have been made in understanding platelet-collagen interactions including the molecular cloning of GPVI, the generation of mouse strains lacking individual collagen receptors, and the development of collagen receptor-specific antibodies and synthetic peptides. It is now recognized that platelet adhesion to collagen requires prior activation of integrins through "inside-out" signals generated by GPVI and reinforced by released second-wave mediators adenosine diphosphate (ADP) and thromboxane A2. These developments have led to revision of the original "2-site, 2-step" model, which now places GPVI in a central position in the complex processes of platelet tethering, activation, adhesion, aggregation, degranulation, and procoagulant activity on collagen. This review discusses these recent developments and proposes possible mechanisms for how GPVI acts in concert with other receptors and signaling pathways to initiate hemostasis and arterial thrombosis.

Bernard-soulier syndrome with a homozygous 13 base pair deletion in the signal peptide-coding region of the platelet glycoprotein Ib(beta) gene

We report a family with Bernard-Soulier syndrome with a homozygous mutation within the GPIb(beta) gene. The proband was a 24-year-old Japanese male who has suffered from life-long bleeding tendency. The patient's sister also had severe bleeding episodes. The proband and the affected sister had no apparent complications including organic or skeletal anomaly, or mental disturbance. They had thrombocytopenia [(35-40) x 10(9)/l] with giant platelets. In addition to platelet size, electron microscopic analysis revealed abnormalities in the internal structures of platelets. Ristocetin-induced platelet aggregation was defective. Flow cytometric analysis and western blot analysis showed that glycoprotein IX was nearly absent in platelets, whereas GPIb(alpha) and GPV were detectable. Genetic studies revealed a 13 base pair deletion in the signal peptide-coding sequence of GPIb(beta). The deletion would cause a frame-shift, resulting in the appearance of a stop codon following an indifferent polypeptide sequence. Analysis of platelet RNA showed that the mutant GPIb(beta) gene was transcribed. The propositus and his affected sister were homozygous for the deletion, whereas their unaffected father and mother were heterozygotes. The molecular defects of this family would help understand the relevance of GPIb(beta) for complex formation of the glycoprotein Ib/IX/V receptor.

Platelet receptors and their role in diseases

The absence or deficiency of specific platelet glycoprotein receptors has a well-defined role in causing several rare bleeding disorders such as Bernard-Soulier syndrome or Glanzmann's thrombasthenia. Several new rare disorders caused by defects in other receptors or their signalling pathways have recently been described. Platelet receptors are also often targets for antibodies in pathological conditions. The roles of platelet receptors or their polymorphism variants in diseases such as cardiovascular disorders have started to be intensively investigated over the last 5 years. Many of these findings still remain controversial. Recent evidence points to a fundamental role for platelets and their receptors in the origins of atherosclerosis. Studies on the role of platelet receptors in diseases such as asthma, diabetes and HIV are still at an early stage.

The alpha(2) integrin subunit-deficient mouse: a multifaceted phenotype including defects of branching morphogenesis and hemostasis

The alpha(2)beta(1) integrin is a collagen/laminin receptor expressed on platelets, endothelial cells, fibroblasts, and epithelial cells. To define the role of the alpha(2)beta(1) integrin in vivo, we created a genetically engineered mouse in which expression of the alpha(2)beta(1) integrin was completely eliminated. Mice deficient in the alpha(2)beta(1) integrin are viable, fertile, and develop normally with no excess lethality of homozygotes. Both alpha(2)beta(1)-integrin protein and alpha(2) mRNA were undetectable in the alpha(2)-null mice. Gross and histological evaluation of the heart, lungs, kidneys, gastrointestinal tract, pancreas, skin, and reproductive tracts revealed no abnormalities. However, quantitative analysis of mammary gland branching morphogenesis demonstrated that branching complexity is markedly diminished in the alpha(2)-deficient animals. Studies in the alpha(2)-deficient animals do not support the proposed roles for the alpha(2)beta(1) integrin on fibroblasts and keratinocytes in wound healing. When compared to platelets from wild-type littermates, platelets from alpha(2)-null mice failed to adhere to type I collagen under either static or shear-stress conditions. Although platelets from alpha(2)-deficient animals aggregated in response to collagen, they did so with prolonged lag time and lessened intensity. The alpha(2)beta(1) integrin-null mouse thus exhibits diverse, sometimes subtle, phenotypes consistent with the widespread pattern of alpha(2)beta(1) integrin expression.