Cytoskeletal regulation of the platelet glycoprotein Ib/V/IX–von Willebrand factor interaction

A Novel Mutation in GP1BB Reveals the Role of the Cytoplasmic Domain of GPIbβ in the Pathophysiology of Bernard-Soulier Syndrome and GPIb-IX Complex Assembly

Bernard-Soulier syndrome (BSS) is an autosomal-recessive bleeding disorder caused by biallelic variants in the GP1BA, GP1BB, and GP9 genes encoding the subunits GPIbα, GPIbβ, and GPIX of the GPIb-IX complex. Pathogenic variants usually affect the extracellular or transmembrane domains of the receptor subunits. We investigated a family with BSS caused by the homozygous c.528_550del (p.Arg177Serfs*124) variant in GP1BB, which is the first mutation ever identified that affects the cytoplasmic domain of GPIbβ. The loss of the intracytoplasmic tail of GPIbβ results in a mild form of BSS, characterized by only a moderate reduction of the GPIb-IX complex expression and mild or absent bleeding tendency. The variant induces a decrease of the total platelet expression of GPIbβ; however, all of the mutant subunit expressed in platelets is correctly assembled into the GPIb-IX complex in the plasma membrane, indicating that the cytoplasmic domain of GPIbβ is not involved in assembly and trafficking of the GPIb-IX receptor. Finally, the c.528_550del mutation exerts a dominant effect and causes mild macrothrombocytopenia in heterozygous individuals, as also demonstrated by the investigation of a second unrelated pedigree. The study of this novel GP1BB variant provides new information on pathophysiology of BSS and the assembly mechanisms of the GPIb-IX receptor.

Proteolytic processing of platelet receptors

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

Membrane skeleton orchestrates the platelet glycoprotein (GP) Ib-IX complex clustering and signaling

Platelet glycoprotein Ib‐IX complex is affixed to the membrane skeleton through interaction with actin binding protein 280 (ABP‐280). We find that removal of the ABP‐280 binding sites in GP Ibα cytoplasmic tail has little impact on the complex clustering induced by antibody crosslinking. However, large truncation of the GP Ibα cytoplasmic tail allows the formation of larger patches of the complex, suggesting that an ABP‐280 independent force may exist. Besides, we observe that the signaling upon GP Ib‐IX clustering is elicited in both membrane lipid domain dependent and independent manner, a choice that relies on how the membrane skeleton interacts with the complex. Our findings suggest a more complex mechanism for how the membrane skeleton regulates the GP Ib‐IX function. © 2016 The Authors IUBMB Life published by Wiley Periodicals, Inc. on behalf of International Union of Biochemistry and Molecular Biology, 68(10):823–829, 2016

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.

Novel antiplatelet activity of protocatechuic acid through the inhibition of high shear stress-induced platelet aggregation

Bleeding is the most common and serious adverse effect of currently available antiplatelet drugs. Many efforts are being made to develop novel antithrombotic agents without bleeding risks. Shear stress-induced platelet aggregation (SIPA), which occurs under abnormally high shear stress, plays a crucial role in the development of arterial thrombotic diseases. Here, we demonstrate that protocatechuic acid (PCA), a bioactive phytochemical from Lonicera (honeysuckle) flowers, selectively and potently inhibits high shear (>10,000 s(-1))-induced platelet aggregation. In isolated human platelets, PCA decreased SIPA and attenuated accompanying platelet activation, including intracellular calcium mobilization, granule secretion, and adhesion receptor expression. The anti-SIPA effect of PCA was mediated through blockade of von Willebrand factor binding to activated glycoprotein Ib, a primary and initial event for the accomplishment of SIPA. Conspicuously, PCA did not inhibit platelet aggregation induced by other endogenous agonists like collagen, thrombin, or ADP that are important in both pathological thrombosis and normal hemostasis. Antithrombotic effects of PCA were confirmed in vivo in a rat arterial thrombosis model, where PCA significantly delayed the arterial occlusion induced by FeCl(3). Of particular note, PCA did not increase bleeding times in a rat tail transection model, whereas conventional antiplatelet drugs, aspirin, and clopidogrel substantially prolonged it. Collectively, these results suggest that PCA may be a novel antiplatelet agent that can prevent thrombosis without increasing bleeding risks.

A novel interaction between FlnA and Syk regulates platelet ITAM-mediated receptor signaling and function

Filamin A (FlnA) cross-links actin filaments and connects the Von Willebrand factor receptor GPIb-IX-V to the underlying cytoskeleton in platelets. Because FlnA deficiency is embryonic lethal, mice lacking FlnA in platelets were generated by breeding FlnA^loxP/loxP females with GATA1-Cre males. FlnA^loxP/y GATA1-Cre males have a macrothrombocytopenia and increased tail bleeding times. FlnA-null platelets have decreased expression and altered surface distribution of GPIbα because they lack the normal cytoskeletal linkage of GPIbα to underlying actin filaments. This results in ∼70% less platelet coverage on collagen-coated surfaces at shear rates of 1,500/s, compared with wild-type platelets. Unexpectedly, however, immunoreceptor tyrosine-based activation motif (ITAM)- and ITAM-like–mediated signals are severely compromised in FlnA-null platelets. FlnA-null platelets fail to spread and have decreased α-granule secretion, integrin αIIbβ3 activation, and protein tyrosine phosphorylation, particularly that of the protein tyrosine kinase Syk and phospholipase C–γ2, in response to stimulation through the collagen receptor GPVI and the C-type lectin-like receptor 2. This signaling defect was traced to the loss of a novel FlnA–Syk interaction, as Syk binds to FlnA at immunoglobulin-like repeat 5. Our findings reveal that the interaction between FlnA and Syk regulates ITAM- and ITAM-like–containing receptor signaling and platelet function.

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.

Clot Stabilization for the Prevention of Bleeding

Coagulation is a finely tuned sequence of reactions beginning with the interaction between tissue factor (TF) and its substrate, factor VII (FVII), and resulting in the formation of a fibrin clot localized to the site of vascular endothelial disruption. While important for fibrin clot formation, thrombin also plays a role in stabilizing the clot against premature fibrinolysis by activating thrombin activatable fibrinolysis inhibitor (TAFI) and factor XIII (FXIII), the terminal enzyme in the coagulation cascade. Despite use of antifibrinolytic agents in various types of surgery to inhibit clot lysis. thereby limiting blood loss and patient exposure to allogeneic blood products, numerous patients still require transfusions for nonsurgical bleeding. This article describes new concepts of localized hemostasis, a potential role for clot stabilization, and inhibition of fibrinolysis for control of bleeding.

Translocation of GPIb and Fc receptor gamma-chain to cytoskeleton in mucetin-activated platelets

Recent studies have implied that GPIb-IX-V as well as functioning as an adhesion receptor may also induce signaling to mediate binding of platelets to damaged vessel wall to prevent bleeding. Reorganization of the cytoskeleton and redistribution of platelet structural proteins and signaling molecules are thought to be important in this early activation process, though the molecular mechanisms remain to be fully defined. In this study, we have used mucetin, a snake venom lectin protein that activates platelets via GPIb, to study the redistribution of GPIb in platelets. In unstimulated platelets, a minor portion of GPIb localized to Triton-insoluble cytoskeleton fractions (TIC). This portion increased considerably after platelet activation by mucetin. We also find increased contents of the FcRgamma chain in TIC. Anti-GPIb antibodies, mocarhagin or cytochalasin D completely inhibited the cytoskeletal translocation. In addition, BAPTA-AM, a cytoplasmic calcium chelator, strongly inhibited this process. On the other hand, inhibitors of alphaIIbbeta3, PLCgamma, PKC, tyrosine kinases, ADP receptor, PI3-kinase or EDTA are effective in preventing GPIb relocation in convulxin- but not in mucetin-activated platelets. We propose that cytoskeletal translocation of GPIb is upstream of alphaIIbbeta3 activation and cross-linking of GPIb is sufficient to induce this event in mucetin-activated platelets.

Identification of a unique filamin A binding region within the cytoplasmic domain of glycoprotein Ibalpha

Binding of the platelet GPIb/V/IX (glycoprotein Ib/V/IX) receptor to von Willebrand factor is critical for platelet adhesion and aggregation under conditions of rapid blood flow. The adhesive function of GPIbalpha is regulated by its anchorage to the membrane skeleton through a specific interaction with filamin A. In the present study, we examined the amino acid residues within the cytoplasmic tail of GPIbalpha, which are critical for association with filamin A, using a series of 25-mer synthetic peptides that mimic the cytoplasmic tail sequences of wild-type and mutant forms of GPIbalpha. Peptide binding studies of purified human filamin A have demonstrated a major role for the conserved hydrophobic stretch L567FLWV571 in mediating this interaction. Progressive alanine substitutions of triple, double and single amino acid residues within the Pro561-Arg572 region suggested an important role for Trp570 and Phe568 in promoting GPIbalpha binding to filamin A. The importance of these two residues in promoting filamin A binding to GPIbalpha in vivo was confirmed from the study of Chinese-hamster ovary cells expressing GPIbalpha Trp570-->Ala and Phe568-->Ala substitutions. Phenotypic analysis of these cell lines in flow-based adhesion studies revealed a critical role for these residues in maintaining receptor anchorage to the membrane skeleton and in maintaining cell adhesion to a von Willebrand factor matrix under high-shear conditions. These studies demonstrate a novel filamin A binding motif in the cytoplasmic tail of GPIbalpha, which is critically dependent on both Trp570 and Phe568.

Signalling through the platelet glycoprotein Ib-V–IX complex

The glycoprotein Ib-V-IX is one of the major adhesive receptors expressed on the surface of circulating platelets. It is composed of four different polypeptides-GPIbalpha, GPIbbeta, GPIX, and GPV-and represents a multifunctional receptor able to interact with a number of ligands, including the adhesive protein von Willebrand factor, the coagulation factors thrombin, factors XI and XII, and the membrane glycoproteins P-selectin and Mac-1. Interaction of GPIb-V-IX with the subendothelial von Willebrand factor is essential for primary haemostasis, as it initiates platelet adhesion to the subendothelial matrix at the sites of vascular injury even under high flow conditions. Upon interaction with von Willebrand factor, GPIb-V-IX initiates transmembrane signalling events for platelet activation, which eventually result in integrin alpha(IIb)beta(3) stimulation and platelet aggregation. The investigation of the biochemical mechanisms for platelet activation by GPIb-V-IX has attracted increasing attention during the last years. This review will describe and discuss recent findings that have provided new insights into the events underlying GPIb-V-IX transmembrane signalling. In particular, it will summarise basic concepts on the structure of this receptor, extracellular ligands, and intracellular interactors potentially involved in transmembrane signalling. The recently suggested role of membrane Fc receptors in GPIb-V-IX-initiated platelet activation will also be discussed, along with the involvement of lipid metabolising enzymes, tyrosine kinases, and the cytoskeleton in the crosstalk between GPIb-V-IX and integrin alpha(IIb)beta(3).

Cell-to-cell variability in the differentiation program of human megakaryocytes

Differentiation of CD34(+) stem/progenitor cells into megakaryocytes is thought to be a uniform, unidirectional process, in which cells transform step by step from less differentiated precursor stages to more differentiated megakaryocytes. Here we propose the concept and present evidence based on single-cell analysis that differentiation occurs along multiple, partially asynchronous routes. In all CD34(+) cells cultured with thrombopoietin, surface appearance of glycoprotein IIIa (GPIIIa) preceded that of GPIb, indicating that the expression of these glycoproteins occurs in a timely ordered manner. Cellular F-actin content increased in parallel with GPIb expression. Only cells that expressed GPIb were polyploid, pointing to co-regulation of GPIb expression, actin cytoskeleton formation and polyploidization during megakaryocytopoiesis. On the other hand, most progenitor cells responded to thrombin but not to thromboxane A(2) analogue by rises in cytosolic [Ca(2+)](i). The appearance of thromboxane-induced responses during megakaryocytopoiesis was not strictly linked to glycoprotein expression, because cells showed responsiveness either before or after GPIb expression. The same non-strictly sequential pattern was observed for disappearance of the Ca(2+) response by prostacyclin mimetic; in some megakaryocytes it occurred before and in others after GPIb expression. Thus, megakaryocytic differentiation follows along independent routes that are either strictly sequential (GPIIIa and GPIb expression) or proceed at different velocities (Ca(2+) signal regulation).

Redistribution of glycoprotein Ib within platelets in response to protease-activated receptors 1 and 4: roles of cytoskeleton and calcium

Thrombin activates human platelets by hydrolyzing the protease-activated receptors PAR-1 and PAR-4, exposing new N-terminal sequences which act as tethered ligands, and binding to glycoprotein (GP) Ib, whose surface accessibility transiently decreases when platelets are stimulated by the enzyme. In an attempt to better understand this latter process, we used the peptides SFLLRNPNDKYEPF (PAR-1-AP or TRAP) and AYPGKF (PAR-4-AP) to study whether hydrolysis of both PAR receptors leads to GPIb redistribution. Both peptides induced surface clearance of GPIb with a maximum at 2 min and 5 min for PAR-1-AP and PAR-4-AP, respectively, followed by a slow return to the surface with levels normalizing between 30 and 60 min. Translocation was associated with the formation of clusters of GPIb as revealed by fluorescence microscopy. This transient redistribution of GPIb was blocked by cytochalasin D and in large part by the membrane permeable Ca2+ chelator, BAPTA. The inhibitor of phosphatidylinositol 3-kinase and myosin light chain kinase, wortmannin, did not significantly modify internalization of GPIb, although its return to the surface was delayed for PAR-1-AP. PAR receptor-mediated association of GPIb to the insoluble cytoskeleton was blocked by cytochalasin D, while BAPTA alone increased and stabilized the presence of GPIb. Globally, immunoprecipitation experiments and analysis of the cytoskeleton confirmed that GPIb translocation is powered by a contractile mechanism involving Ca2+ mobilization, actin polymerization, and myosin incorporation into the cytoskeleton and that both PAR-1 and PAR-4 can activate this process.

Filamin A binding to the cytoplasmic tail of glycoprotein Ibalpha regulates von Willebrand factor-induced platelet activation

We examined the hypothesis that filamin A binding to the cytoplasmic tail of platelet glycoprotein Ibalpha (GpIbalpha) is regulated by pathologic shear stress and modulates von Willebrand factor (VWF)-induced platelet activation. To begin, we examined filamin binding to GpIbalpha in Chinese hamster ovary cells coexpressing mutant human GpIb-IX and wild-type human filamin A. We observed that many different deletions and truncations N-terminal to GpIbalpha's cytoplasmic domain residue 594 disrupted filamin A binding, but that binding was unaffected by 14 different point mutations in hydrophilic residues between amino acids 557 and 593. To try to narrow GpIbalpha's filamin A-binding domain, we next measured the effect of several cytoplasmic domain peptides on human filamin A binding to a GST-GpIbalpha cytoplasmic domain fusion protein. One peptide (residues 557-575; designated "A4 peptide") inhibited filamin A binding to the GST-GpIbalpha cytoplasmic domain fusion protein and competed with GpIbalpha for binding to filamin A. When the A4 peptide was delivered to intact human platelets using a carrier peptide, we observed the dose-dependent inhibition of VWF-induced platelet aggregation in response to both ristocetin and shear stress. The effect of the A4 peptide on shear-induced platelet aggregation was accompanied by the attenuation of shear-induced filamin A binding to GpIbalpha and diminished shear-dependent protein tyrosine phosphorylation. These results suggest that shear-dependent VWF-induced platelet activation affects filamin A binding to GpIb-IX-V, and that filamin A binding to the cytoplasmic tail of GpIbalpha regulates proaggregatory tyrosine kinase signaling.

Signaling role for phospholipase C gamma 2 in platelet glycoprotein Ib alpha calcium flux and cytoskeletal reorganization. Involvement of a pathway distinct from FcR gamma chain and Fc gamma RIIA

Interaction of the platelet GPIb-V-IX complex with surface immobilized von Willebrand factor (vWf) is required for the capture of circulating platelets and their ensuing activation. In previous work, it was found that GPIb/vWf-mediated platelet adhesion triggers Ca2+ release from intracellular stores, leading to cytoskeletal reorganization and filopodia extension. Despite the potential functional importance of GPIb-induced cytoskeletal changes, the signaling mechanisms regulating this process have remained ill-defined. The studies presented here demonstrate an important role for phospholipase C (PLC)-dependent phosphoinositide turnover for GPIb-dependent cytoskeletal remodeling. This is supported by the findings that the vWf-GPIb interaction induced a small increase in inositol 1,4,5-triphosphate (IP3) and that treating platelets with the IP3 receptor antagonist APB-2 or the PLC inhibitor U73122 blocked cytosolic Ca2+ flux and platelet shape change. Normal shape change was observed in G alpha q-/- mouse platelets, excluding a role for PLC beta isoforms in this process. However, decreased shape change and Ca2+ mobilization were observed in mice lacking PLC gamma 2, demonstrating that this isotype played an important, albeit incomplete, role in GPIb signaling. The signaling pathways utilized by GPIb involved one or more members of the Src kinase family as platelet shape change and Ca2+ flux were inhibited by the Src kinase inhibitors PP1 and PP2. Strikingly, shape change and Ca2+ release occurred independently of immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors, because these platelet responses were normal in human platelets treated with the anti-Fc gamma RIIA blocking monoclonal antibody IV.3 and in mouse platelets deficient in the FcR gamma chain. Taken together, these studies define an important role for PLC gamma 2 in GPIb signaling linked to platelet shape change. Moreover, they demonstrate that GPIb-dependent calcium flux and cytoskeletal reorganization involves a signaling pathway distinct from that utilized by ITAM-containing receptors.

Platelets and von Willebrand factor

The interaction of platelets with von Willebrand factor (VWF) is crucial in the initiation of any hemostatic or thrombotic process. VWF enables the platelet, via its surface glycoprotein receptors, to adhere to exposed subendothelium and to respond to shear stress in the blood. Via VWF that is stored and released from platelet alpha-granules and from Weibel-Palade bodies of endothelial cells, the hemostatic system can respond locally to lesions in the vessel wall and can initiate the activation of other platelets. This review describes the molecular structure of VWF, its functions and its interactions with the platelet membrane glycoprotein receptors GP Ib-IX-V and GP IIb-IIIa. As well, the role of VWF in shear-induced platelet adhesion and aggregation is described, and mechanisms are discussed that control the size of VWF multimers and the responsiveness of platelets to multimeric VWF. Finally, the review discusses the role of locally released VWF from platelet alpha-granules and from Weibel-Palade bodies for platelet activation in neonates and adults.

Cytoplasmic truncation of glycoprotein Ib alpha weakens its interaction with von Willebrand factor and impairs cell adhesion

The interaction of the platelet glycoprotein (GP) Ib-IX-V complex with von Willebrand factor (VWF) is a critical step in the adhesion of platelets to the subendothelial matrix following endothelial cell damage, particularly under arterial flow conditions. In the human GP Ib-IX-V complex, the recognition of VWF appears to be mediated entirely by GP Ibalpha, the largest of four GP Ib-IX-V polypeptides. The goal of the present study was to investigate the involvement of the cytoplasmic domain of GP Ibalpha in the GP Ib-IX-VWF interaction under both static conditions and in the presence of high fluid shear stress. Using Chinese hamster ovary (CHO) cells that express GP Ibbeta, GP IX, and either wild-type GP Ibalpha or GP Ibalpha mutants missing various lengths of the cytoplasmic domain, we evaluated adhesion and flow-driven cell rolling on immobilized VWF in a parallel-plate flow chamber. Cells expressing GP Ibalpha polypeptides with truncations of 6-82 amino acids rolled faster than cells expressing wild-type GP Ibalpha. Cells that expressed polypeptides with intact actin-binding protein 280 binding sites (truncated to residue 582 of 610) rolled more slowly than those expressing GP Ibalpha with longer truncations. The rolling velocity of cells expressing truncated GP Ibalpha mutants increased with decreasing VWF coating density. In addition, a fraction of the truncated cells exhibited saltatory translocation at the lower VWF densities. Studies measuring the GP Ibalpha-VWF bond strength of three of the mutants using laser tweezers showed that progressive deletion of the cytoplasmic domain led to progressive weakening of the strength of individual GP Ibalpha-VWF bonds.

von Willebrand factor (VWF)-dependent human platelet activation: porcine VWF utilizes different transmembrane signaling pathways than does thrombin to activate platelets, but both require protein phosphatase function

The interaction between von Willebrand factor (VWF) and glycoprotein (GP) Ib results in platelet agglutination and activation of many signaling intermediates. To determine if VWF-dependent platelet activation requires the participation of pivotal transmembrane signaling pathways, we analyzed VWF-dependent platelet activation profiles following inhibition of several transmembrane signaling intermediates. This was accomplished using porcine VWF, which has been shown to interact with human GPIb independently of shear stress or ristocetin. Platelet alpha (CD62) and lysozomal granule release (CD63), microparticle formation, and platelet agglutination/aggregation were evaluated. The ability of signaling inhibitors to prevent VWF-dependent platelet activation was compared to their ability to inhibit thrombin-dependent activation. The results demonstrate that VWF-dependent platelet activation can occur independently of the activities of protein kinase C (PKC), wortmannin-sensitive phosphatidylinositide 3-kinase, and phospholipase C, as well as independently of elevations in the concentration of intracellular calcium. In sharp contrast, these transmembrane signaling intermediates are required for thrombin-dependent platelet activation. In addition, thrombin-dependent but not VWF-dependent platelet activation was associated with elevations in the concentration of intracellular calcium under the conditions used. The family of signaling intermediates which appeared to be pivotal for both thrombin- and VWF-dependent platelet activation were the protein tyrosine phosphatases and the serine/threonine phosphatases. It is concluded that thrombin-dependent platelet activation relies on the activation of several transmembrane signaling pathways, whereas VWF-dependent platelet activation is dependent upon the activity of protein phosphatases. Inhibition of these phosphatases in vivo may provide a novel therapeutic approach for treating VWF-dependent platelet disorders such as thrombotic thrombocytopenic purpura or arterial thrombosis.

Glycoprotein Ib cross-linking/ligation on echicetin-coated surfaces or echicetin-IgMkappa in stirred suspension activates platelets by cytoskeleton modulated calcium release

Cross-linking platelet GPIb with the snake C-type lectin echicetin provides a specific technique for activation via this receptor. This allows GPIb-dependent mechanisms to be studied without the necessity for shear stress-induced binding of von Willebrand factor or primary alpha(IIb)beta(3) involvement. We already showed that platelets are activated, including tyrosine phosphorylation, by echicetin-IgMkappa-induced GPIb cross-linking. We now investigate the mechanism further and demonstrate that platelets, without modulator reagents, spread directly on an echicetin-coated surface, by a GPIb-specific mechanism, causing exocytosis of alpha-granule markers (P-selectin) and activation of alpha(IIb)beta(3). This spreading requires actin polymerization and release of internal calcium stores but is not dependent on external calcium nor on src family tyrosine kinases. Cross-linking of GPIb complex molecules on platelets, either in suspension or via specific surface attachment, is sufficient to induce platelet activation.

Genetic abnormalities of Bernard-Soulier syndrome

Bernard-Soulier Syndrome (BSS) is an autosomal recessive bleeding disorder due to quantitative or qualitative abnormalities in the glycoprotein (GP) Ib/IX/V complex, the platelet receptor for von Willebrand factor. BSS is characterized by giant platelets, thrombocytopenia, and prolonged bleeding time, and the hallmark of this disorder is the absence of ristocetin-induced platelet agglutination. In the last 10 years, the molecular and genetic bases of many GPIb/IX/V defects have been elucidated, providing a better understanding of primary hemostasis and structure-function relations of the complex. Thus far, more than 30 mutations of the GPIbalpha, GPIbbeta, or GPIX genes have been described in BSS. Recent studies also have shown that the phenotypes caused by mutations in the subunits of the GPIb/IX/V span a wide spectrum, from the normal phenotype, to isolated giant platelet disorders/macrothrombocytopenia, to full-blown BSS and platelet-type von Willebrand disease. Although recent progress in molecular biology has clarified the genotype-phenotype relationships of the GPIb/IX/V disorders, a close examination of platelet morphology on blood smears is still indispensable for a proper diagnosis. In this review, we summarize recent advances in the molecular basis of BSS with special emphasis on giant platelets and the genetic characteristics of Japanese BSS.

Site-directed mutagenesis of platelet glycoprotein Ib alpha demonstrating residues involved in the sulfation of tyrosines 276, 278, and 279

The interaction between platelet glycoprotein (GP) Ib alpha and von Willebrand factor (VWF) is essential for initiation of hemostasis. The sulfation of the 3 tyrosine residues 276, 278, and 279 in GPIb alpha is an important posttranslational modification that seems to promote the interaction with VWF. The environment where sulfation of tyrosines occurs has been proposed to contain highly acidic residues. This investigation has examined the highly acidic region from Asp249 to Asp287 in the mature GPIb alpha protein. Changes to most of the carboxylic acids in this region resulted in decreased reactivity to VWF. Only 3 mutants (Glu270Gln, Asp283Asn, Asp283Asn/Glu285Gln/Asp287Asn) resulted in the abolition of sulfation. Two novel mutations were also created. First, a deletion of the 7 amino acids from Tyr276 to Glu282 led to a loss of sulfation and totally abolished VWF binding in the presence of botrocetin. This confirms that it is these 3 tyrosines that undergo sulfation and that this region is crucial for botrocetin-mediated VWF binding. The second mutation involves changing the lysine residues at 253, 258, and 262 to alanine. This also led to distinct changes in VWF binding and abolition of sulfation.

RhoA sustains integrin alpha IIbbeta 3 adhesion contacts under high shear

The small GTPase RhoA modulates the adhesive nature of many cell types; however, despite high levels of expression in platelets, there is currently limited evidence for an important role for this small GTPase in regulating platelet adhesion processes. In this study, we have examined the role of RhoA in regulating the adhesive function of the major platelet integrin, alpha(IIb)beta(3). Our studies demonstrate that activation of RhoA occurs as a general feature of platelet activation in response to soluble agonists (thrombin, ADP, collagen), immobilized matrices (von Willebrand factor (vWf), fibrinogen) and high shear stress. Blocking the ligand binding function of integrin alpha(IIb)beta(3), by pretreating platelets with c7E3 Fab, demonstrated the existence of integrin alpha(IIb)beta(3)-dependent and -independent mechanisms regulating RhoA activation. Inhibition of RhoA (C3 exoenzyme) or its downstream effector Rho kinase had no effect on integrin alpha(IIb)beta(3) activation induced by soluble agonists or adhesive substrates, however, both inhibitors reduced shear-dependent platelet adhesion on immobilized vWf and shear-induced platelet aggregation in suspension. Detailed analysis of the sequential adhesive steps required for stable platelet adhesion on a vWf matrix under shear conditions revealed that RhoA did not regulate platelet tethering to vWf or the initial formation of integrin alpha(IIb)beta(3) adhesion contacts but played a major role in sustaining stable platelet-matrix interactions. These studies define a critical role for RhoA in regulating the stability of integrin alpha(IIb)beta(3) adhesion contacts under conditions of high shear stress.

Lateral clustering of platelet GP Ib-IX complexes leads to up-regulation of the adhesive function of integrin alpha IIbbeta 3

Binding of von Willebrand factor (VWF) to GP Ib-IX mediates initial platelet adhesion and increases the subsequent adhesive function of alpha(IIb)beta(3). Because these responses are promoted most effectively by large VWF multimers, we hypothesized that receptor clustering modulates GP Ib-IX function. To test this, GP IX was fused at its cytoplasmic tail to tandem repeats of FKBP, and GP Ib-IX(FKBP)(2) and alpha(IIb)beta(3) were expressed in Chinese hamster ovary cells. Under flow conditions at wall shear rates of up to 2000 s(-1), GP Ib-IX(FKBP)(2) mediated cell tethering to immobilized VWF, just as in platelets. Conditional oligomerization of GP Ib-IX(FKBP)(2) by AP20187, a cell-permeable FKBP dimerizer, caused a decrease in cell translocation velocities on VWF (p < 0.001). Moreover, clustering of GP Ib-IX(FKBP)(2) by AP20187 led to an increase in alpha(IIb)beta(3) function, manifested under static conditions by increased cell adhesion to fibrinogen (p < 0.01) and under flow by increased stable cell adhesion to VWF (p < 0.04). Clustering of GP Ib-IX(FKBP)(2) also stimulated rapid tyrosine phosphorylation of ectopically expressed Syk, a putative downstream effector of GP Ib-IX in platelets. These studies establish that GP Ib-IX oligomerization, per se, affects the interaction of this receptor with VWF and its ability to influence the adhesive function of alpha(IIb)beta(3). By extrapolation, GP Ib-IX clustering in platelets may promote thrombus formation.

Interaction between platelet glycoprotein Ibalpha and filamin-1 is essential for glycoprotein Ib/IX receptor anchorage at high shear

The interaction of the glycoprotein (GP) Ib-V-IX receptor complex with the membrane skeleton of platelets is dependent on a specific interaction between the cytoplasmic tail of GPIbalpha and filamin-1. This interaction has been proposed to regulate key aspects of platelet function, including the ligand binding of GPIb-V-IX and the ability of the cells to sustain adhesion to von Willebrand factor (vWf) under high shear. In this study we have examined sequences in the GPIbalpha intracellular domain necessary for interaction of the receptor with filamin-1. We have identified two adjacent sequences involving amino acids 557-568 and 569-579 of the GPIbalpha cytoplasmic domain that are critical for normal association between the receptor complex and filamin-1. Under flow conditions, Chinese hamster ovary (CHO) cells expressing these two mutant receptors exhibited an increase in translocation velocity that was associated with increased cell detachment from the vWf matrix at high shear. The shear-dependent acceleration in velocity of mutant Delta557-568 and Delta569-579 CHO cells was associated with a critical defect in receptor anchorage, evident from significant extraction of GPIb-IX from the CHO cell membrane at high shear. These studies define a critical role for amino acids within the 557-579 sequence of GPIbalpha for interaction with filamin-1.

Functional property of von Willebrand factor under flowing blood

von Willebrand factor (vWF) is produced in megakaryocytes and endothelial cells, is stored in the alpha-granule of platelets and in the Weibel-Palade body of endothelial cells, and is present in plasma and vascular subendothelium. This huge protein with a unique multimeric structure plays a pivotal role in both hemostasis and pathological intravascular thrombosis, in which vWF contributes to both platelet adhesion/aggregation and blood coagulation through its multiple adhesive functions for the platelet membrane receptors, glycoprotein Ib-IX-V complex, integrin alphaIIbbeta3, heparin, various types of collagen, and coagulation factor VIII. Among various functions, the most characteristic feature of vWF is its determinant role on platelet thrombus formation under high-shear-rate conditions. Indeed, at in vivo rheological situations where platelets are flowing with high speed in the bloodstream, the only reaction that can initiate mural thrombogenesis is the interaction of vWF with platelet glycoprotein Ibalpha. The recent x-ray analysis of the crystal structure of various functional domains and functional studies of this protein under experimental flow conditions have rapidly advanced and revised our knowledge of the structure-function relationships of vWF, a key protein for hemostasis and arterial thrombosis.

Inhibition of cytoskeletal assembly by cytochalasin B prevents signaling through tyrosine phosphorylation and secretion triggered by collagen but not by thrombin

Activation of platelets leads to cytoskeletal assembly that is responsible for platelet motility and internal contraction. We have evaluated the involvement of the cytoskeleton in platelet activation by two strong agonists, collagen and thrombin. Activation was assessed by measuring changes in cytoskeletal assembly, externalization of activation-dependent markers and expression of procoagulant activity, and tyrosine phosphorylation of proteins, in both the absence and the presence of cytochalasin B. Activation of platelets with collagen and thrombin induced morphological changes and increased the expression of CD62P, CD63, glycoprotein IV, and binding of annexin V to platelets. Moreover, both activating agents induced actin polymerization, increased the association of other contractile proteins, and promoted tyrosine phosphorylation of multiple proteins, some of which were associated with the cytoskeleton. The presence of cytochalasin B blocked the previous events when collagen was used as the activating agent, although binding of annexin V still occurred. In contrast, platelet response to thrombin was not completely prevented by the presence of cytochalasin B. Thus, activation by collagen requires a functional cytoskeleton to trigger signaling through tyrosine phosphorylation and secretion. This is not the case for thrombin, which is capable of activating signaling mechanisms in the presence of strong inhibitors of cytoskeletal assembly. Moreover, the expression of a procoagulant surface in platelets still occurs even when platelet motility has been inhibited.

Platelet aggregation is not initiated by platelet shape change

Because the initial decrease in light transmission in platelet aggregometry is attributed to platelet shape change, it is widely held that platelet shape change is a prerequisite for platelet aggregation. We conducted this study to determine the basis of this initial optical effect in aggregometry. Platelets were activated with ADP, thrombin, or the thrombin receptor agonist peptide SFLLRN (TRAP(1-6)). In every case the initial decrease in light transmission occurred with the concomitant formation of microaggregates. This was also seen when preactivated platelets, which cannot undergo further morphological changes, were used, and when platelets were activated in the presence of shape-change inhibitors such as cytochalasin D and vincristine. Microscopy analysis of samples fixed at minimum light transmission in the aggregometer, which is generally assumed to signal shape change, always showed the presence of microaggregates. Microaggregation appeared to be distinct from full aggregation, as it was not inhibited by the addition of CD61, an antibody to the beta(3) integrin. To model these findings, fibrinogen-coated latex spheres, which cannot change shape, were aggregated with thrombin; the initial decrease in light transmission was still seen, and microaggregates formed at this time. These results indicate that platelet shape change is not a prerequisite for aggregation and that the signal widely believed to represent shape change reflects platelet microaggregation instead. We conclude that platelet aggregation occurs independently of shape change and that shape change is not necessarily followed by aggregation. These observations suggest an alternative role for platelet shape change of single platelets.

Phenotype changes resulting in high-affinity binding of von Willebrand factor to recombinant glycoprotein Ib-IX: analysis of the platelet-type von Willebrand disease mutations

To maintain hemostasis under shear conditions, there must be an interaction between the platelet glycoprotein (GP) Ib-IX receptor and the plasma ligand von Willebrand factor (vWf). In platelet-type von Willebrand disease (Pt-vWD), hemostasis is compromised. Two mutations in the GPIbalpha polypeptide chain have been identified in these patients-a glycine-233 to valine change and a methionine-239 to valine change. For this investigation, these mutant proteins have been expressed in a Chinese hamster ovary cell model system. Ligand-binding studies were performed at various concentrations of ristocetin, and adhesion assays were performed under flow conditions. The Pt-vWD mutations resulted in a gain-of-function receptor. vWf binding was increased at all concentrations of ristocetin examined, and adhesion on a vWf matrix was enhanced in terms of cell tethering, slower rolling velocity, and decreased detachment with increasing shear rate. Two other mutations were also introduced into the GPIbalpha chain. One mutation, encompassing both the Pt-vWD mutations, created an increase in the hydrophobicity of this region. The second mutation, involving a valine-234 to glycine change, decreased the hydrophobicity of this region. Both mutations also resulted in a gain-of-function receptor, with the double mutation producing a hyperreactive receptor for vWf. These data further support the hypothesis that ligand binding is regulated by conformational changes in the amino-terminal region of GPIbalpha, thereby influencing the stability of the GPIbalpha-vWf interaction.

Mechanisms of platelet aggregation

Platelet aggregation is initiated by receptor activation coupled to intracellular signaling leading to activation of integrin alphaIIbbeta3. Recent advances in the study of platelet receptors for collagen, von Willebrand factor, thrombin, and adenosine diphosphate are providing new insights into the mechanisms of platelet aggregation.

Glycoprotein Ib/IX/V binding to the membrane skeleton maintains shear-induced platelet aggregation

The extracellular domain of glycoprotein (Gp) Ibalpha serves as the von Willebrand factor (vWf) receptor that triggers shear stress-dependent platelet aggregation. Its intracellular domain associates with actin-binding protein-280 (filamin 1a) that binds directly to filamentous actin, thereby linking the membrane skeleton to GpIbalpha. We examined the functional significance of GpIbalpha interactions with actin during platelet aggregation in response to 120 dyn/cm(2) shear stress. Lysates of resting and sheared platelets were centrifuged at approximately 13,000xg for 15 min, and GpIbalpha was immunoprecipitated from the lysate supernatant. GpIbalpha and coimmunoprecipitated proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotted with antibodies specific for GpIbalpha and actin. We observed a significant increase in the amounts of actin coimmunoprecipitating with GpIbalpha as platelets aggregated in response to shear stress. Actin/GpIbalpha interactions reached a maximum after 90 s of shear stress. Monoclonal antibody (mAb) blockade of vWf binding to GpIbalpha inhibited shear stress-induced platelet aggregation and actin associating with GpIbalpha. Pretreatment of platelets with cytochalasin D resulted in the inhibition of actin binding to GpIbalpha in sheared platelets and in an increase in the rate and magnitude of platelet disaggregation. These data indicate that shear stress causes changes in the association between GpIbalpha and the actin-based membrane skeleton. The increased interaction between GpIbalpha and the actin-based membrane skeleton results from shear-induced vWf binding to GpIbalpha and is mechanoprotective in that it maintains shear-induced aggregation of activated platelets.