Convulxin binds to native, human glycoprotein Ib alpha

RUNX1-deficient human megakaryocytes demonstrate thrombopoietic and platelet half-life and functional defects

Heterozygous defects in runt-related transcription factor 1 (RUNX1) are causative of a familial platelet disorder with associated myeloid malignancy (FPDMM). Because RUNX1-deficient animal models do not mimic bleeding disorder or leukemic risk associated with FPDMM, development of a proper model system is critical to understanding the underlying mechanisms of the observed phenotype and to identifying therapeutic interventions. We previously reported an in vitro megakaryopoiesis system comprising human CD34+ hematopoietic stem and progenitor cells that recapitulated the FPDMM quantitative megakaryocyte defect through a decrease in RUNX1 expression via a lentiviral short hairpin RNA strategy. We now show that shRX-megakaryocytes have a marked reduction in agonist responsiveness. We then infused shRX-megakaryocytes into immunocompromised NOD scid gamma (NSG) mice and demonstrated that these megakaryocytes released fewer platelets than megakaryocytes transfected with a nontargeting shRNA, and these platelets had a diminished half-life. The platelets were also poorly responsive to agonists, unable to correct thrombus formation in NSG mice homozygous for a R1326H mutation in von Willebrand Factor (VWFR1326H), which switches the species-binding specificity of the VWF from mouse to human glycoprotein Ibα. A small-molecule inhibitor RepSox, which blocks the transforming growth factor β1 (TGFβ1) pathway and rescued defective megakaryopoiesis in vitro, corrected the thrombopoietic defect, defects in thrombus formation and platelet half-life, and agonist response in NSG/VWFR1326H mice. Thus, this model recapitulates the defects in FPDMM megakaryocytes and platelets, identifies previously unrecognized defects in thrombopoiesis and platelet half-life, and demonstrates for the first time, reversal of RUNX1 deficiency-induced hemostatic defects by a drug.

South American snake venoms with abundant neurotoxic components. Composition and toxicological properties. A literature review

In South America there are three snake genera with predominantly neurotoxic venoms: Crotalus, Micrurus and Hydrophis, which include nine species/subspecies, 97 species and a single marine species, respectively. Although accidents with neurotoxic venoms are less frequent than those with anticoagulant, cytotoxic or necrotic venoms (e.g. from Bothrops), they are of major public health importance. Venoms from genus Crotalus have been extensively studied, while data on the venoms from the other two genera are very limited, especially for Hydrophis. The venoms of North and South American Crotalus species show biochemical and physiopathological differences. The former species cause bothrops-like envenomation symptoms, while the latter mainly have neurotoxic and myotoxic effects, leading to respiratory paralysis and, occasionally, renal failure by myoglobinuria and death, often with no local lesions. Micrurus and Hydrophis also cause neurotoxic envenomations. Many studies have isolated, identified and characterized new enzymes and toxins, thus expanding the knowledge of snake venom composition. The present review summarizes the currently available information on neurotoxic venoms from South American snakes, with a focus on protein composition and toxicological properties. It also includes some comments concerning potential medical applications of elapid and crotalic toxins.

Acquired platelet GPVI receptor dysfunction in critically ill patients with sepsis

Glycoprotein VI (GPVI), the platelet immunoreceptor tyrosine activating motif (ITAM) receptor for collagen, plays a striking role on vascular integrity in animal models of inflammation and sepsis. Understanding ITAM-receptor signaling defects in humans suffering from sepsis may improve our understanding of the pathophysiology, especially during disease onset. In a pilot study, platelets from 15 patients with sepsis were assessed consecutively at day of admission, day 5 to 7, and the day of intensive care unit (ICU) discharge and subjected to comprehensive analyses by flow cytometry, aggregometry, and immunoblotting. Platelet function was markedly reduced in all patients. The defect was most prominent after GPVI stimulation with collagen-related peptide. In 14 of 15 patients, GPVI dysfunction was already present at time of ICU admission, considerably before the critical drop in platelet counts. Sepsis platelets failed to transduce the GPVI-mediated signal to trigger tyrosine phosphorylation of Syk kinase or LAT. GPVI deficiency was partially inducible in platelets of healthy donors through coincubation in whole blood, but not in plasma from patients with sepsis. Platelet aggregation upon GPVI stimulation increased only in those patients whose condition ameliorated. As blunted GPVI signaling occurred early at sepsis onset, this defect could be exploited as an indicator for early sepsis diagnosis, which needs to be confirmed in prospective studies.

Effect of Isolated Proteins from Crotalus Durissus Terrificus Venom on Leishmania (Leishmania) Amazonensis-Infected Macrophages

BACKGROUND

Cutaneous and mucocutaneous leishmaniasis are parasitic diseases characterized by skin manifestations. In Brazil, Leishmania (Leishmania) amazonensis is one of the etiological agents of cutaneous leishmaniasis. The therapeutic arsenal routinely employed to treat infected patients is unsatisfactory, especially for pentavalent antimonials, as they are often highly toxic, poorly tolerated and of variable effectiveness. This study aimed to evaluate in vitro the leishmanicidal activity of toxins isolated from Crotalus durissus terrificus venom as a new approach for the treatment of leishmaniasis.

METHODS

The comparative effects of crotamine, crotoxin, gyrotoxin, convulxin and PLA2 on bone marrow-derived macrophages infected with L. (L.) amazonensis as well as the release of TGF-β from the treated macrophages were studied.

RESULTS AND DISCUSSION

Crotamine had the strongest inhibitory effect on parasite growth rate (IC50: 25.65±0.52 μg/mL), while convulxin showed the weakest inhibitory effect (IC50: 52.7±2.21 μg/mL). In addition, TGF-β was significantly reduced after the treatment with all toxins evaluated.

CONCLUSION

The Crotalus durissus terrificus toxins used in this study displayed significant activity against L. (L.) amazonensis, indicating that all of them could be a potential alternative for the treatment of cutaneous leishmaniasis.

Influence of long-term proteasome inhibition on platelet responsiveness mediated by bortezomib

INTRODUCTION

Although platelets contain a full proteasome system, its role in platelet function is not completely understood yet. Since the proteasome system may be involved in time-delayed processes, platelet responsiveness was investigated after long-term, bortezomib-mediated proteasome inhibition.

MATERIALS AND METHODS

Citrate-anticoagulated whole blood was stored with 5 nM and 1 μM bortezomib for 24 h. Consecutively, aggregation was measured by light transmission in platelet-rich-plasma (PRP). Flow cytometry was performed to determine phosphorylation levels of the vasodilator-stimulated phosphoprotein (VASP), fibrinogen binding, PAC1-antibody binding and purinergic receptor expression in PRP, P2Y12 activity or glycoprotein (GP) Ib and IIb expression in whole blood. P2Y1 and P2X1 activities were assessed by calcium flux-induced fluorescence in washed platelets. Using PRP, adherent platelets on fibrinogen-, collagen- and ristocetin-coated surfaces were visualized and quantified by immunostaining.

RESULTS

Under bortezomib, VASP phosphorylation was less inducible and nitric oxide-induced inhibition of fibrinogen binding was slightly reduced. Proteasome inhibition did not tamper adenosine diphosphate-mediated aggregation or purinergic receptor expression and activity. Induced expression of activated fibrinogen receptors and fibrinogen binding were not significantly influenced by incubation with bortezomib for 24 h. Aggregation values with threshold agonist concentrations were increased under bortezomib. Despite unchanged GPIb expression, bortezomib-treated platelets showed enhanced adhesion on coated surfaces.

CONCLUSIONS

In platelets incubated for 24 h, bortezomib mediates a slight attenuation of inhibitory signaling, associated with facilitated platelet aggregation using threshold agonist concentrations and enhanced adhesion on agonist-coated surfaces.

Structurally Robust and Functionally Highly Versatile-C-Type Lectin (-Related) Proteins in Snake Venoms

Snake venoms contain an astounding variety of different proteins. Among them are numerous C-type lectin family members, which are grouped into classical Ca²⁺- and sugar-binding lectins and the non-sugar-binding snake venom C-type lectin-related proteins (SV-CLRPs), also called snaclecs. Both groups share the robust C-type lectin domain (CTLD) fold but differ in a long loop, which either contributes to a sugar-binding site or is expanded into a loop-swapping heterodimerization domain between two CLRP subunits. Most C-type lectin (-related) proteins assemble in ordered supramolecular complexes with a high versatility of subunit numbers and geometric arrays. Similarly versatile is their ability to inhibit or block their target molecules as well as to agonistically stimulate or antagonistically blunt a cellular reaction triggered by their target receptor. By utilizing distinct interaction sites differentially, SV-CLRPs target a plethora of molecules, such as distinct coagulation factors and receptors of platelets and endothelial cells that are involved in hemostasis, thrombus formation, inflammation and hematogenous metastasis. Because of their robust structure and their high affinity towards their clinically relevant targets, SV-CLRPs are and will potentially be valuable prototypes to develop new diagnostic and therapeutic tools in medicine, provided that the molecular mechanisms underlying their versatility are disclosed.

Trowaglerix Venom Polypeptides As a Novel Antithrombotic Agent by Targeting Immunoglobulin-Like Domains of Glycoprotein VI in Platelet

Objective—

Currently prescribed antiplatelet drugs have 1 common side effect—an increased risk of hemorrhage and thrombocytopenia. On the contrary, bleeding defects associated with glycoprotein VI (GPVI) expression deficiency are usually slightly prolonged bleeding times. However, GPVI antagonists are lacking in clinic.

Approach and Results—

Using reverse-phase high-performance liquid chromatography and sequencing, we revealed the partial sequence of trowaglerix α subunit, a potent specific GPVI-targeting snaclec (snake venom C-type lectin protein). Hexapeptide (Troα6 [trowaglerix a chain hexapeptide, CKWMNV]) and decapeptide (Troα10) derived from trowaglerix specifically inhibited collagen-induced platelet aggregation through blocking platelet GPVI receptor. Computational peptide design helped to design a series of Troα6/Troα10 peptides. Protein docking studies on these decapeptides and GPVI suggest that Troα10 was bound at the lower surface of D1 domain and outer surface of D2 domain, which was at the different place of the collagen-binding site and the scFv (single-chain variable fragment) D2-binding site. The newly discovered site was confirmed by inhibitory effects of polyclonal antibodies on collagen-induced platelet aggregation. This indicates that D2 domain of GPVI is a novel and important binding epitope on GPVI-mediated platelet aggregation. Troα6/Troα10 displayed prominent inhibitory effect of thrombus formation in fluorescein sodium-induced platelet thrombus formation of mesenteric venules and ferric chloride-induced carotid artery injury thrombosis model without prolonging the in vivo bleeding time.

Conclusions—

We develop a novel antithrombotic peptides derived from trowaglerix that acts through GPVI antagonism with greater safety—no severe bleeding. The binding epitope of polypeptides on GPVI is novel and important. These hexa/decapeptides have therapeutic potential for developing ideal small-mass GPVI antagonists for arterial thrombogenic diseases.

von Willebrand factor mutation promotes thrombocytopathy by inhibiting integrin αIIbβ3

von Willebrand disease type 2B (vWD-type 2B) is characterized by gain-of-function mutations in von Willebrand factor (vWF) that enhance its binding to the glycoprotein Ib-IX-V complex on platelets. Patients with vWD-type 2B have a bleeding tendency that is linked to loss of vWF multimers and/or thrombocytopenia. In this study, we uncovered evidence that platelet dysfunction is a third possible mechanism for bleeding tendency. We found that platelet aggregation, secretion, and spreading were diminished due to inhibition of integrin αIIbβ3 in platelets from mice expressing a vWD-type 2B-associated vWF (vWF/p.V1316M), platelets from a patient with the same mutation, and control platelets pretreated with recombinant vWF/p.V1316M. Impaired platelet function coincided with reduced thrombus growth. Further, αIIbβ3 activation and activation of the small GTPase Rap1 were impaired by vWF/p.V1316M following exposure to platelet agonists (thrombin, ADP, or convulxin). Conversely, thrombin- or ADP-induced Ca2+ store release, which is required for αIIbβ3 activation, was normal, indicating that vWF/p.V1316M acts downstream of Ca2+ release and upstream of Rap1. We found normal Syk phosphorylation and PLCγ2 activation following collagen receptor signaling, further implying that vWF/p.V1316M acts directly on or downstream of Ca2+ release. These data indicate that the vWD-type 2B mutation p.V1316M is associated with severe thrombocytopathy, which likely contributes to the bleeding tendency in vWD-type 2B.

Megakaryocyte-specific deletion of the protein-tyrosine phosphatases Shp1 and Shp2 causes abnormal megakaryocyte development, platelet production, and function

The SH2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2 have been implicated in regulating signaling from a variety of platelet and megakaryocyte receptors. In this study, we investigate the functions of Shp1 and Shp2 in megakaryocytes and platelets. Megakaryocyte/platelet (MP)-specific deletion of Shp1 in mice resulted in platelets being less responsive to collagen-related peptide due to reduced GPVI expression and signaling via the Src family kinase (SFK)-Syk-PLCγ2 pathway, and fibrinogen due to reduced SFK activity. By contrast, deletion of Shp2 in the MP lineage resulted in macrothrombocytopenia and platelets being hyper-responsive to anti-CLEC-2 antibody and fibrinogen. Shp1- and Shp2-deficient megakaryocytes had partial blocks at 2N/4N ploidy; however, only the latter exhibited reduced proplatelet formation, thrombopoietin, and integrin signaling. Mice deficient in both Shp1 and Shp2 were severely macrothrombocytopenic and had reduced platelet surface glycoprotein expression, including GPVI, αIIbβ3, and GPIbα. Megakaryocytes from these mice were blocked at 2N/4N ploidy and did not survive ex vivo. Deletion of the immunoreceptor tyrosine-based inhibition motif-containing receptor G6b-B in the MP lineage phenocopied multiple features of Shp1/2-deficient mice, suggesting G6b-B is a critical regulator of Shp1 and Shp2. This study establishes Shp1 and Shp2 as major regulators of megakaryocyte development, platelet production, and function.

An acquired defect associated with abnormal signaling of the platelet collagen receptor glycoprotein VI

INTRODUCTION

Ligands acting at the platelet collagen receptor, glycoprotein (GP)VI, induce intracellular FcRγ/Syk-dependent signaling pathways and Syk-dependent or Syk-independent generation of intracellular reactive oxygen species (ROS). Additional signaling-dependent or signaling-independent pathways lead to metalloproteinase-mediated shedding of GPVI.

AIM

Analysis of platelet GPVI expression and signaling in a patient with a collagen-selective defect associated with myelodysplastic syndrome (MDS) uniquely demonstrates divergent pathways leading to ROS generation and Syk phosphorylation in human platelets.

METHODS

Surface expression of GPVI and ligand-induced ROS generation was quantitated by flow cytometry. GPVI shedding and Syk phosphorylation were analyzed by Western blot.

RESULTS

Despite platelet count/size and GPVI surface expression within normal ranges, platelet-rich plasma showed no aggregation in response to collagen or GPVI-selective agonist collagen-related peptide, but aggregated in response to other agonists, consistent with dysfunctional GPVI signaling. We observed rapid GPVI-dependent Syk-independent ROS generation and disulfide-dependent GPVI homodimerization, but not Syk-dependent ROS or ligand-induced shedding. Temporal analysis showed a gradual decline in platelet count and the appearance of ligand-induced phosphorylation of an ∼40-kDa Syk fragment.

CONCLUSIONS

These studies show that GPVI ligation in platelets induces intracellular ROS production independent of either Syk activation or divergent pathways leading to platelet aggregation or ectodomain shedding.

GPIbα regulates platelet size by controlling the subcellular localization of filamin

Interaction between the cytoplasmic domain of GPIbα with its cytoskeletal binding partner, filamin, is a major determinant of platelet size, and deficiency of either protein results in macrothrombocytopenia. To clarify the mechanism by which GPIbα-filamin interactions regulate platelet production, we manipulated the expression levels of filamin and GPIb in cultured embryonic stem cells (ESCs) that were subsequently differentiated into platelets. Knocking down filamins A and B resulted in the production of ESC-derived proplatelets with abnormally large swellings and proplatelet shafts that generated giant platelets in culture. Large platelets could also be generated by overexpressing GPIbα in ESCs, or by overexpressing in vivo a transgene encoding a chimeric protein containing the cytoplasmic domain of GPIbα. To identify the mechanism by which the GPIb:filamin ratio regulates platelet size, we manipulated filamin and GPIbα levels in HEK293T cells and examined the effects of overexpressing either protein on their ability to traffic to the cell periphery. Accumulation of either protein within the endoplasmic reticulum resulted in trapping of the other. Taken together, these data demonstrate that coordinated expression of GPIbα and filamin is required for efficient trafficking of either protein to the cell surface, and for production of normal-sized platelets.

Haemostatically active proteins in snake venoms

Snake venom proteins that affect the haemostatic system can cause (a) lowering of blood coagulability, (b) damage to blood vessels, resulting in bleeding, (c) secondary effects of bleeding, e.g. hypovolaemic shock and organ damage, and (d) thrombosis. These proteins may, or may not, be enzymes. We review the data on the most relevant haemostatically active proteinases, phospholipases A₂, L-amino acid oxidases and 5'-nucleotidases from snake venoms. We also survey the non-enzymatic effectors of haemostasis from snake venoms--disintegrins, C-type lectins and three-finger toxins. Medical applications have already been found for some of these snake venom proteins. We describe those that have already been approved as drugs to treat haemostatic disorders or are being used to diagnose such health problems. No clinical applications, however, currently exist for the majority of snake venom proteins acting on haemostasis. We conclude with the most promising potential uses in this respect.

Reflective Random Indexing and indirect inference: a scalable method for discovery of implicit connections

The discovery of implicit connections between terms that do not occur together in any scientific document underlies the model of literature-based knowledge discovery first proposed by Swanson. Corpus-derived statistical models of semantic distance such as Latent Semantic Analysis (LSA) have been evaluated previously as methods for the discovery of such implicit connections. However, LSA in particular is dependent on a computationally demanding method of dimension reduction as a means to obtain meaningful indirect inference, limiting its ability to scale to large text corpora. In this paper, we evaluate the ability of Random Indexing (RI), a scalable distributional model of word associations, to draw meaningful implicit relationships between terms in general and biomedical language. Proponents of this method have achieved comparable performance to LSA on several cognitive tasks while using a simpler and less computationally demanding method of dimension reduction than LSA employs. In this paper, we demonstrate that the original implementation of RI is ineffective at inferring meaningful indirect connections, and evaluate Reflective Random Indexing (RRI), an iterative variant of the method that is better able to perform indirect inference. RRI is shown to lead to more clearly related indirect connections and to outperform existing RI implementations in the prediction of future direct co-occurrence in the MEDLINE corpus.

GPIb potentiates GPVI-induced responses in human platelets

Platelet adhesion to vascular subendothelial proteins at the site of blood vessel injury is critical for initiating haemostasis. Collagen is a major matrix protein that binds plasma von Willebrand factor (vWF) when the endothelium becomes damaged and therefore in vivo platelets are likely to encounter both of these agonists simultaneously, through glycoprotein VI (GPVI) and alpha2beta1 receptors for collagen and GPIb-V-IX and alphaIIbbeta3 receptors for vWF. We hypothesised a potentiatory role for vWF that would synergise with collagen leading to functional activation and show this to be the case for platelet aggregation, 5-HT secretion and calcium responses. Synergy between these two agonists is likely to involve receptors GPVI and GPIb-V-IX, for collagen and vWF, respectively, since 5-HT secretion in response to collagen is potentiated by vWF in the presence of either EGTA or EDTA, which prevent binding to integrins alphaIIbbeta3 (EGTA) or both alphaIIbbeta3 and alpha2beta1 (EDTA). In addition, vWF is also able to potentiate 5-HT secretion responses to collagen-related peptide, confirming that GPVI is able to support synergy with vWF. These findings are important in that they reveal a novel role for vWF in platelet activation as a potentiator of platelet activation by collagen.

A snake venom metalloproteinase, kistomin, cleaves platelet glycoprotein VI and impairs platelet functions

BACKGROUND AND OBJECTIVES

Injuries to the vessel wall and subsequent exposure of the matrix of the subendothelial layer resulted in thrombus formation. Platelet glycoprotein (GP) Ib and VI play a crucial role in matrix-induced activation and aggregation of platelets.

METHODS AND RESULTS

In the present study, we reported that the GPIb-cleaving snake venom metalloproteinase (SVMP), kistomin, inhibited collagen-induced platelet aggregation. Moreover, kistomin inhibited platelet aggregation induced by convulxin (CVX, a GPVI agonist) and a GPVI-specific antibody in a concentration and time-dependent manner. Kistomin treatment decreased platelet GPVI but not integrin alpha2beta1 and alphaIIbbeta3, accompanied with the formation of GPVI cleavage fragments, as determined by flow cytometric and Western blot analyses. In addition, intact platelet GPVI and recombinant GPVI were digested by kistomin to release 25- and 35-kDa fragments, suggesting that kistomin cleaved GPVI near the mucin-like region. We designed four synthetic peptides ranging from Leu180 to Asn249 as the substrates for kistomin and found that kistomin cleaved these synthetic peptides at FSE205/A206TA and NKV218/F219TT, as analyzed by MALDI-TOF-MS. In addition, GPVI-specific antibody-induced tyrosine kinase phosphorylation in platelets was reduced after kistomin pretreatment, and platelet adhesion to collagen but not to fibrinogen was attenuated by kistomin.

CONCLUSIONS

We provided here the first evidence that a P-I snake venom metalloproteinase, kistomin, inhibits the interaction between collagen and platelet GPVI through its proteolytic activity on GPVI, thus providing an alternative strategy for developing new anti-thrombotic agents.

A familial platelet function disorder associated with abnormal signalling through the glycoprotein VI pathway

The platelet collagen receptor, glycoprotein (GP)VI, of the immunoreceptor family forms a complex with the von Willebrand factor (VWF) receptor, GPIb-IX-V, critical for initiating thrombus formation. GPVI is co-associated with Fc receptor gamma-chain (FcRgamma), which contains a cytoplasmic immunoreceptor tyrosine-based activation motif domain, involved in activation of Syk, and a signalling cascade leading to (i) activation of alpha(IIb)beta(3), which binds VWF and fibrinogen and mediates platelet aggregation, and (ii) metalloproteinase-mediated shedding of the GPVI ectodomain (blocked by Syk inhibitors), a key mechanism for regulating GPVI surface expression. In this study, we report a familial case of abnormal platelet aggregation with dysfunctional signalling through GPVI that uniquely demonstrates divergent alpha(IIb)beta(3)-activating and GPVI-shedding pathways. The patient is a 60-year-old female with a history of immune disorders, excessive bleeding from childhood and a life-threatening haemorrhage post-trauma. Platelet aggregation to ADP, thrombin receptor-agonist peptide or ristocetin/VWF was normal (indicating normal expression and function of alpha(IIb)beta(3)), but platelet aggregation to GPVI agonists, collagen, collagen-related peptide, or convulxin, was defective. Both GPVI/FcRgamma expression and ligand-induced GPVI ectodomain shedding were normal, confirming expression of functional GPVI/FcRgamma, but suggesting a signalling defect downstream of Syk. A genetic defect in GPVI/Fcgamma signalling compromising platelet function is hypothesised in this family.

Lebectin and lebecetin, two C-type lectins from snake venom, inhibit α5β1 and αv-containing integrins

Integrins are essential protagonists in the complex multistep process of cancer progression and metastasis. We recently reported that lebectin, a novel C-type lectin from Macrovipera lebetina venom, displays an anti-integrin activity. In this study, we extend this observation to lebecetin, a second C-type lectin isolated from the same venom and previously reported as a potent inhibitor of platelet aggregation. Both venom lectins appear to exert their effect on cell adhesion, migration, invasion and proliferation by inhibiting alpha5beta1 and alphav-containing integrins. Moreover, the inhibition of alpha5beta1 and alphav integrins is likely due to the binding of venom peptides, as both lebectin and lebecetin co-immunoprecipitate with these integrins. Lebectin and lebecetin are thus the first examples of venom C-type lectins inhibiting an integrin other than the collagen receptor alpha2beta1.

Glycoprotein VI agonists have distinct dependences on the lipid raft environment

BACKGROUND

It has been reported that the association of glycoprotein VI (GPVI) with lipid rafts regulates GPVI signaling in platelets.

OBJECTIVE

Secreted adenosine 5'-diphosphate (ADP) potentiates GPVI-induced platelet aggregation at particular agonist concentrations. We have investigated whether the decrease in GPVI signaling, previously reported in platelets with disrupted rafts, is a result of the loss of agonist potentiation by ADP.

METHODS

We disrupted platelet lipid rafts with methyl-beta-cyclodextrin and measured signaling events downstream of GPVI activation.

RESULTS

Lipid raft disruption decreases aggregation induced by low concentrations of convulxin, but this decrease is almost eliminated in the presence of ADP antagonists. Signaling indicators, such as protein phosphorylation and calcium mobilization, were not affected by raft disruption in collagen or convulxin stimulated platelets. Interestingly, however, raft disruption directly reduced GPVI signaling induced by collagen-related peptide.

CONCLUSIONS

Lipid rafts do not directly contribute to signaling by the physiologic agonist collagen. The effects of disruption of lipid rafts in in vitro assays can be attributed to inhibition of ADP feedback that potentiates GPVI signaling.

Differential inhibitory effects of platelet glycoprotein IIb/IIIa antagonists on aggregation induced by procoagulant agonists

INTRODUCTION

In addition to mediating the final common pathway of aggregation, the glycoprotein (GP) IIb/IIIa receptor participates in the activation of coagulation on the platelet surface. High-affinity conformation of GP IIb/IIIa in response to collagen-induced inside-out signalling seems to be mediated by GP VI(-FcRgamma) and reinforced by release of soluble mediators.

METHODS

We assessed the effects of the three currently available GP IIb/IIIa antagonists--abciximab, tirofiban and eptifibatide--on platelet aggregation induced by various procoagulant and GP VI-related agonists, i.e. collagen-related peptide (CRP), convulxin and collagen fibrils, in PPACK-anticoagulated platelet-rich plasma.

RESULTS

At concentrations that equally inhibited 80% of ADP-induced maximal aggregation abciximab-inhibited GP VI-mediated platelet responses to CRP or convulxin significantly more than the low-molecular-weight antagonists (CRP: abciximab 75+/-18%, tirofiban 41+/-7% and eptifibatide 41+/-6%; convulxin: abciximab 90+/-6%, tirofiban 64+/-20%, eptifibatide 61+/-14%, p<0.01 for all). In contrast, aggregation induced by collagen was equally abolished with all antagonists under the similar conditions. During CRP- or convulxin-triggered platelet activation, inhibition of fibrin polymerisation with GPRP potentiated the antiaggregatory effects of tirofiban and eptifibatide to reach that of abciximab. GPRP as such did not affect platelet aggregation.

CONCLUSIONS

GP IIb/IIIa antagonists exhibit distinct inhibition profiles in platelet aggregation, depending on fibrin polymerization and calcium. Specifically, the ability of procoagulant platelet agonists to expose pre-activated and ligand-bound GP IIb/IIIa from the internal pool seems important.

Effect of multimer size and a natural dimorphism on the binding of convulxin to platelet glycoprotein (GP)VI

BACKGROUND

Convulxin (CVX), a C-type lectin from the venom of Crotalus durissus terrificus, is a potent activator of human platelets, binding predominantly to glycoprotein (GP)VI. Native CVX is an octamer composed of four alphabeta-heterodimers [(alphabeta)(4)]. Two different native sequences have been reported, one bearing lysine (K), the other glutamic acid (E), at beta chain residue 89, but the physiological relevance of this difference is unknown.

OBJECTIVE

We used the Drosophila S2 system to express recombinant CVX (rCVX) heterodimers (alphabeta) and site-directed mutagenesis to evaluate the influence of multimer size and the substitution betaK89E on CVX function.

METHODS

By flow cytometry, native CVX and both recombinant forms bind to human platelets in whole blood. By surface plasmon resonance (BIAcore, Piscataway, NJ, USA), the calculated equilibrium dissociation constants (K(D)) were: rCVX alphabeta89K, 11.3 x 10(-8) m; rCVX alphabeta89E, 9 x 10(-8) m; and native CVX, 2.8 x 10(-8) m.

RESULTS

Thus, the affinities of the two rCVX forms for human, recombinant GPVI are essentially the same, but the relative affinity of native CVX is about 3-fold higher. The minimum concentration of native CVX that induces maximal human platelet aggregation (70 pm) is roughly 400-fold lower than that of either rCVX (29 nm).

CONCLUSIONS

These results are consistent with the hypothesis that the ability of the native CVX octamer to cluster mobile GPVI molecules within the platelet membrane may be the single most important factor that contributes to the efficiency with which CVX is able to induce platelet activation.

Blood cells as targets of snake toxins

Snake venoms are mixtures of enzymes and peptides which exert toxicological effects by targeting their substrates or receptors upon envenomation. Snake venom proteins widely affect vascular system including circulating blood cells, coagulation factors, and vascular wall components. Many of the toxic proteins have multiple targets. For example, some metalloproteinase domain-containing snake venom protein cleaves not only fibrinogen but also receptors on platelets. Also, it is frequent that toxins from different snake venom protein families are capable of binding to a common target on cells. Most of the cytotoxic effects in the venom are usually results of the activities of metalloproteinase, C-type lectin, disintegrin, cysteine-rich protein, as well as phospholipase A(2). There has been a growing interest in studying the structure and function of these snake venom proteins because many of them have high structural homologies to proteins found in human. Therefore, the understanding of how these toxins interact with their targets may contribute to the discovery of novel physiological processes and the development of therapeutic agents for cardiovascular diseases. In this review, we summarize how snake toxins target blood cells with an emphasis on their effects on platelet function.

GPVI and integrin alphaIIb beta3 signaling in platelets

This review summarizes recent developments in our understanding of the molecular basis of platelet activation by two distinct types of surface receptor, the immunoglobulin GPVI, and the integrin alphaIIb beta3 (also known as GPIIbIIIa). These two classes of receptor signal through similar yet distinct tyrosine kinase-based signaling cascades leading to activation of phospholipase C gamma2. The significance of these signaling cascades in platelet adhesion and platelet aggregation at arterial rates of shear is discussed.

Snake venoms and hemostasis

Snake venoms are complex mixtures of biologically active proteins and peptides. Many of them affect hemostasis by activating or inhibiting coagulant factors or platelets, or by disrupting endothelium. Based on sequence, these snake venom components have been classified into various families, such as serine proteases, metalloproteinases, C-type lectins, disintegrins and phospholipases. The various members of a particular family act selectively on different blood coagulation factors, blood cells or tissues. For almost every factor involved in coagulation or fibrinolysis there is a venom protein that can activate or inactivate it. Venom proteins affect platelet function by binding or degrading vWF or platelet receptors, activating protease-activated receptors or modulating ADP release and thromboxane A2 formation. Some venom enzymes cleave key basement membrane components and directly affect capillary blood vessels to cause hemorrhaging. L-Amino acid oxidases activate platelets via H2O2 production.

A C-type lectin-like protein from Agkistrodon acutus venom binds to both platelet glycoprotein Ib and coagulation factor IX/factor X

Agkisacutacin, a C-type lectin-like protein (CLP) isolated from Agkistrodon acutus venom, had been previously identified as an antagonist of platelet aggregation induced by ristocetin, as well as a certain extent fibrinogenlytic activity. In this study, agkisacutacin was further purified by three-step chromatography, and its biological functions were characterized. Agkisacutacin after further purification retained the effect on ristocetin-induced, von Willebrand factor-dependent platelet aggregation, while it lost the fibrinogenlytic activity. FACS and ELISA assays showed that agkisacutacin belongs to membrane glycoprotein Ib-binding protein (GPIb-bp) for it could block and inhibit the binding of anti-GPIb antibody to GPIb. Especially, agkisacutacin exhibits anti-coagulant activity and the function as IX/X-binding protein was confirmed by PAGE and ELISA. So, agkisacutacin is the first reported CLP that binds to both platelet membrane receptors and coagulation factors.

Snake venom C-type lectins interacting with platelet receptors. Structure–function relationships and effects on haemostasis

Snake venoms contain components that affect the prey either by neurotoxic or haemorrhagic effects. The latter category affect haemostasis either by inhibiting or activating platelets or coagulation factors. They fall into several types based upon structure and mode of action. A major class is the snake C-type lectins or C-type lectin-like family which shows a typical folding like that in classic C-type lectins such as the selectins and mannose-binding proteins. Those in snake venoms are mostly based on a heterodimeric structure with two subunits alpha and beta, which are often oligomerized to form larger molecules. Simple heterodimeric members of this family have been shown to inhibit platelet functions by binding to GPIb but others activate platelets via the same receptor. Some that act via GPIb do so by inducing von Willebrand factor to bind to it. Another series of snake C-type lectins activate platelets by binding to GPVI while yet another series uses the integrin alpha(2)beta(1) to affect platelet function. The structure of more and more of these C-type lectins have now been, and are being, determined, often together with their ligands, casting light on binding sites and mechanisms. In addition, it is relatively easy to model the structure of the C-type lectins if the primary structure is known. These studies have shown that these proteins are quite a complex group, often with more than one platelet receptor as ligand and although superficially some appear to act as inhibitors, in fact most function by inducing thrombocytopenia by various routes. The relationship between structure and function in this group of venom proteins will be discussed.

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.

Snake venom probes of platelet adhesion receptors and their ligands

Snake venom proteins that modulate platelet adhesive interactions are chiefly from either of two main structural families: the C-type lectin-like family, or the metalloproteinase-disintegrins. Snake venom probes from both families selectively target platelet adhesion receptors, including glycoprotein (GP) Ib-IX-V, GP VI, alpha2beta1 and alphaIIbbeta3 (GP IIb-IIIa). These receptors act together to mediate platelet adhesion, activation and aggregation (thrombus formation) under hydrodynamic shear stress in flowing blood. The receptors are members of the leucine-rich repeat family (GP Ib-IX-V), the immunoglobulin superfamily (GP VI), or integrins (alpha2beta1, alphaIIbbeta3). In addition, adhesive glycoproteins in matrix and/or plasma such as von Willebrand factor (that binds GP Ibalpha and alphaIIbbeta3), collagen (that binds GP V, GP VI and alpha2beta1), or fibrinogen (that binds alphaIIbbeta3), are also targeted by C-type lectin family or metalloproteinase-disintegrin snake venom proteins. Emerging structural and functional evidence is beginning to explain how interactions between the conserved structural module-domains that make up these mammalian and snake proteins are regulated. Whether homologous adhesion/counter-receptors on platelets and other vascular cells are also potential snake venom targets is as yet largely unexplored.

Platelet adhesion and signaling induced by the octapeptide primary binding sequence (KOGEOGPK) from type III collagen

Platelet adhesion to vascular collagens is an essential step in the initiation of hemostasis and thrombosis. Several platelet receptors interact with type I and type III collagens, including GP Ia/IIa and GP VI. We recently described a new platelet receptor (TIIICBP) specific for a type III collagen-related primary binding sequence, the KOGEOGPK octapeptide. Here, we characterize platelet adhesion to the immobilized octapeptide and demonstrate that this adhesion 1) is Ca2+ and Mg2+ independent, suggesting a noninvolvement of GP Ia/IIa; 2) is not inhibited by an antibody against GP VI; and 3) triggers platelet protein tyrosine phosphorylation. Whereas TXA2 has minimal effects, released ADP via only P2Y12 potentiates platelet adhesion to the octapeptide. Octapeptide-induced platelet adhesion triggers platelet signaling through tyrosine phosphorylation of the 68 kDa subunit of TIIICBP, Syk, PLCgamma2, and FAK. Tyrosine phosphorylation of the FcR gamma-chain and LAT is also observed but to a lesser extent than with type III collagen, suggesting the requirement of GP VI for full tyrosine phosphorylation of FcR gamma-chain and LAT. The present study provides evidence for a critical role for the type III collagen-related KOGEOGPK octapeptide in mediating platelet adhesion and signaling, and consequently in platelet-collagen interactions.-

Molecular diversity and accelerated evolution of C-type lectin-like proteins from snake venom

A number of C-type lectin-like proteins that affect thrombosis and hemostasis by inhibiting or activating specific platelet membrane receptors or blood coagulation factors have been isolated from the venom of various snake species and characterized and more than 80 have been sequenced. Recent data on the primary sequences and 3D structures of C-type lectins and C-type lectin-like proteins from snake venoms have enabled us to analyze their molecular evolution. Statistical analysis of their cDNA sequences shows that C-type lectin-like proteins, with some exceptions, have evolved in an accelerated manner to acquire their diverse functions. Phylogenetic analysis shows that the A and B chains of C-type lectin-like proteins are clearly separated from C-type lectins and that the A and B chains are further divided into a group of platelet receptor-binding proteins and a group of coagulation factor-binding proteins. Elucidation of the tertiary structures of several C-type lectin-like proteins led to the discovery of a unique domain-swapping interaction between heterodimeric subunits, which creates a concave surface for ligand binding.

Dysfunctional platelet membrane receptors: from humans to mice

Insights into hemostasis and thrombosis have historically benefited from the astute diagnosis of human bleeding and thrombotic disorders followed by decades of careful biochemical characterization. This work has set the stage for the development of a number of mouse models of hemostasis and thrombosis generated by gene targeting strategies in the mouse genome. The utility of these models is the in depth analysis that can be performed on the precise molecular interactions that support hemostasis and thrombosis along with efficacy testing of various therapeutic strategies. Already the mouse has proven to be an excellent model of the processes that support hemostasis and thrombosis in the human vasculature. A brief summary of the salient phenotypes from knockout mice missing key platelet receptors is presented, including the glycoprotein (GP) Ib-IX-V and GP IIb/IIIa (alphaIIb/beta3) receptors; the collagen receptors, GP VI and alpha2/beta1; the protease activated receptors (PARs); and the purinergic receptors, P2Y(1) and P2Y(12). A few differences exist between mouse and human platelets and where appropriate those will be highlighted in this review. Concluding remarks focus on the importance of understanding the power and limitations of various in vitro, ex vivo and in vivo models currently being used and the impact of the mouse strain on the described platelet phenotype.

Crystallization and preliminary X-ray crystallographic analysis of agkicetin-C from Deinagkistrodon acutus venom

The crystallization and preliminary crystallographic analysis of agkicetin-C, a well known platelet glycoprotein Ib (GPIb) antagonist from the venom of Deinagkistrodon acutus found in Anhui Province, China is reported. Crystals of agkicetin-C suitable for structure determination were obtained from 1.8 M ammonium sulfate, 40 mM MES pH 6.5 with 2%(v/v) PEG 400. Interestingly, low buffer concentrations of MES seem to be necessary for crystal growth. The crystals of agkicetin-C belong to space group C2, with unit-cell parameters a = 177.5, b = 97.7, c = 106.8 A, beta = 118.5 degrees, and diffract to 2.4 A resolution. Solution of the phase problem by the molecular-replacement method shows that there are four agkicetin-C molecules in the asymmetric unit, with a VM value of 3.4 A3 Da(-1), which corresponds to a high solvent content of approximately 64%. Self-rotation function calculations show a single well defined non-crystallographic twofold axis with features that may represent additional elements of non-crystallographic symmetry.

Regulation of platelet membrane levels of glycoprotein VI by a platelet-derived metalloproteinase

Thrombosis can be initiated when activated platelets adhere to injured blood vessels via the interaction of subendothelial collagen with its platelet receptor, glycoprotein (GP) VI. Here we observed that incubation of platelets with convulxin, collagen, or collagen-related peptide (CRP) resulted in GPVI signaling-dependent loss of surface GPVI and the appearance of an approximately 55-kDa soluble fragment of GPVI as revealed by immunoblotting. Ethylenediaminetetraacetic acid (EDTA) or GM6001 (a metalloproteinase inhibitor with broad specificity) prevented this loss. In other receptor systems, calmodulin binding to membrane-proximal cytoplasmic sequences regulates metalloproteinase-mediated ectodomain shedding. In this regard, we have previously shown that calmodulin binds to a positively charged, membrane-proximal sequence within the cytoplasmic tail of GPVI. Incubation of platelets with calmodulin inhibitor W7 (150 μM) resulted in a time-dependent loss of GPVI from the platelet surface. Both EDTA and GM6001 prevented this loss. Surface plasmon resonance demonstrated that W7 specifically blocked the association of calmodulin with an immobilized synthetic peptide corresponding to the calmodulin-binding sequence of GPVI. These findings suggest that disruption of calmodulin binding to receptor cytoplasmic tails by agonist binding to the receptor triggers metalloproteinase-mediated loss of GPVI from the platelet surface. This process may represent a potential mechanism to regulate GPVI-dependent platelet adhesion.

TMVA, a snake C-type lectin-like protein from Trimeresurus mucrosquamatus venom, activates platelet via GPIb

TMVA is a C-type lectin-like protein with potent platelet activating activity from Trimeresurus mucrosquamatus venom. In the absence of von Willebrand factor (vWF), TMVA dose-dependently induced aggregation of washed platelets. Anti-GP Ib monoclonal antibodies (mAbs), HIP1, specifically inhibited TMVA-induced aggregation in a dose-dependent manner. The aggregation was also inhibited by mAb P2 (an anti-GP IIb mAb). Flow cytometric analysis revealed that FITC-TMVA bound to human formalin-fixed platelets in a saturable manner, and its binding was specifically blocked by HIP1 in a dose-dependent manner. Flow cytometric analysis showed that TMVA did not bind to platelet GPIX, GPIIb, GPIIIa, GPIa, GPIIa and GPIV. Moreover, the platelet aggregation induced by TMVA was partially inhibited when platelet was pretreated with mocarhagin, a snake venom protease that specifically cleaves human GPIb. These results suggest that TMVA is a strong platelet agonist via GPIb and it might have multiple functional binding-sites on GPIb molecule or on other unknown receptor.

A role for the thiol isomerase protein ERP5 in platelet function

Formation and rearrangement of disulfide bonds during the correct folding of nascent proteins is modulated by a family of enzymes known as thiol isomerases, which include protein disulfide isomerase (PDI), endoplasmic reticulum protein 5 (ERP5), and ERP57. Recent evidence supports an alternative role for this family of proteins on the surface of cells, where they are involved in receptor remodeling and recognition. In platelets, blocking PDI with inhibitory antibodies inhibits a number of platelet activation pathways, including aggregation, secretion, and fibrinogen binding. Analysis of human platelet membrane fractions identified the presence of the thiol isomerase protein ERP5. Further study showed that ERP5 is resident mainly on platelet intracellular membranes, although it is rapidly recruited to the cell surface in response to a range of platelet agonists. Blocking cell-surface ERP5 using inhibitory antibodies leads to a decrease in platelet aggregation in response to agonists, and a decrease in fibrinogen binding and P-selectin exposure. It is possible that this is based on the disruption of integrin function, as we observed that ERP5 becomes physically associated with the integrin beta(3) subunit during platelet stimulation. These results provide new insights into the involvement of thiol isomerases and regulation of platelet activation.

The regulation of integrin-linked kinase in human platelets: evidence for involvement in the regulation of integrin alpha 2 beta 1

BACKGROUND

Activation of the platelet integrin alpha 2 beta 1 is closely regulated due to the high thrombogenicity of its ligand. As a beta 1 interacting kinase, ILK represents a candidate intracellular regulator of alpha 2 beta 1 in human platelets.

OBJECTIVES

We investigated the regulation of ILK in human platelets and the role of ILK in regulating alpha 2 beta 1 activation in HEL cells, a megakaryocytic cell line.

METHODS

An in-vitro kinase assay was used to determine the effect of platelet agonists on ILK kinase activity together with the contribution of PI3K and PKC on ILK activation. Interaction of ILK with beta 1-integrin subunits was investigated by coimmunoprecipitation and the role of ILK in regulating alpha 2 beta 1 function assessed by overexpression studies in HEL cells.

RESULTS

We report that collagen and thrombin modulate ILK kinase activity in human platelets in an aggregation-independent manner. Furthermore, ILK activity is dually regulated by PI3K and PKC in thrombin-stimulated platelets and regulated by PI3K in collagen-stimulated cells. ILK associates with the beta 1-integrin subunits immunoprecipitated from platelet cell lysates, an association which increased upon collagen stimulation. Overexpression of ILK in HEL cells enhanced alpha 2 beta 1-mediated adhesion whereas overexpression of kinase-dead ILK reduced adhesion, indicating a role for this kinase in the positive regulation of alpha 2 beta 1.

CONCLUSIONS

Our findings that ILK regulates alpha 2 beta 1 in HEL cells, is activated in platelets and associates with beta 1-integrins, raise the possibility that it may play a key role in adhesion events upon agonist stimulation of platelets.

Differential regulation of actin stress fiber assembly and proplatelet formation by alpha2beta1 integrin and GPVI in human megakaryocytes

The actin cytoskeleton plays a major role in platelet function. In contrast, its precise role in the function of megakaryocytes (MKs) is less understood but may be important for a chemoattractive response and an efficient proplatelet formation. In the marrow microenvironment, mature MKs are in contact with the extracellular matrix, including fibrillar collagen type I. MKs express alpha2beta1 integrin and the immunoglobulin superfamily member glycoprotein VI (GPVI), the main receptors for collagen. Using function-blocking antibodies or specific ligands, we investigated in primary human MKs how alpha2beta1 integrin and GPVI regulate stress fiber formation, the primary actin structures needed for cell contraction. Stress fiber assembly requires synergistic activation of the MAPK/Erk1/2 pathway and the small guanosine triphosphatase Rho via its effector, Rho-associated coiled-coil kinase (ROCK). alpha2beta1 integrin is crucial for stress fiber formation, whereas GPVI triggers rapid and sustained activation of the Erk1/2 pathway. Strikingly, after a longer adhesion time, proplatelet formation was significantly inhibited by the engagement of alpha2beta1 integrin, not by GPVI, likely through the Rho/ROCK pathway. Thus, proplatelet formation in human MKs could be tightly regulated by differential interactions with their collagen receptors. We propose that this interaction with collagen prevents proplatelet formation within the marrow.

The P2Y1 receptor plays an essential role in the platelet shape change induced by collagen when TxA2 formation is prevented

ADP and TxA2 are secondary agonists which play an important role as cofactors when platelets are activated by agonists such as collagen or thrombin. The aim of the present study was to characterize the role of the ADP receptor P2Y(1) in collagen-induced activation of washed platelets. Inhibition of P2Y(1) alone with the selective antagonist MRS2179 prolonged the lag phase preceding aggregation in response to low or high concentrations of fibrillar collagen, without affecting the maximum amplitude of aggregation or secretion. A combination of MRS2179 and aspirin resulted in complete inhibition of platelet shape change at low and high collagen concentrations, together with a profound decrease in aggregation and secretion. Scanning electron microscopy showed that these platelets had conserved the discoid morphology typical of the resting state. A lack of shape change was also observed in aspirin-treated P2Y(1)- and G(alphaq)-deficient mouse platelets and in delta-storage pool-deficient platelets from Fawn Hooded rats. In contrast, when the second ADP receptor P2Y(12) was inhibited with AR-C69931MX, aspirin-treated platelets were still able to change shape and displayed only a moderate decrease in aggregation and secretion. In conclusion, this study provides evidence that collagen requires not only the TxA2 receptor Tpalpha, but also P2Y(1), to induce platelet shape change.

Von Willebrand factor present in fibrillar collagen enhances platelet adhesion to collagen and collagen-induced platelet aggregation

We examined the basis of the differences observed between different collagen preparations in their ability to aggregate platelets and support their adhesion under flow. As in previous studies, we found fibrillar collagen to be 10-fold more potent than acid-soluble collagen in inducing platelet aggregation and found that acid-soluble collagen did not support the adhesion of washed platelets under flow. Further, platelets in whole blood adhered to surfaces coated with either fibrillar or acid-soluble collagen, but thrombi formed faster and grew larger on fibrillar collagen. As a possible basis for this difference, we found that fibrillar collagen, but not acid-soluble collagen, contains a substantial quantity of von Willebrand factor (VWF), as demonstrated by enzyme-linked immunosorbent assay and by the ability of fibrillar collagen to support the adhesion of VWF antibody-coated beads and to agglutinate GPIb-IX-V complex-expressing Chinese hamster ovary cells. Supporting a role for VWF in collagen-induced platelet aggregation, aggregation induced by acid-soluble collagen was greatly enhanced by added VWF. Further, platelet aggregation by fibrillar collagen was partially blocked by a GPIbalpha antibody that inhibits the GPIb-VWF interaction. Taken together, these results suggest that much of the difference in prothrombotic potency of different collagens is directly related to their differences in VWF content. This probably accounts for the different conclusions made regarding the relative importance of different direct and indirect collagen receptors in collagen-dependent platelet functions and further emphasizes the close synergistic roles of the GPIb-IX-V complex and the collagen receptors GPVI and alpha2beta1 in supporting platelet adhesion.

Severe deficiency of glycoprotein VI in a patient with gray platelet syndrome

We report a novel case of gray platelet syndrome (GPS) where a severe deficiency of the platelet collagen receptor, glycoprotein (GP) VI, accompanies classical symptoms of a low platelet count and platelets lacking alpha-granules. Dense granules were normally present. Platelet aggregation with collagen was severely decreased, as was the response to convulxin (Cvx), a GPVI agonist. Quantitative analysis of GPVI using fluorescein isothiocyanate (FITC)-Cvx in flow cytometry showed its virtual absence on the patient's platelets. The GPVI deficiency was confirmed using monoclonal antibodies in Western blotting and in immunogold labeling on frozen thin sections where internal pools of GPVI were confirmed for normal platelets. The Fc receptor gamma-chain, constitutively associated with GPVI in normal platelets, was present in subnormal amounts, and the phospholipase C gamma 2-dependent activation pathway appeared to function normally. No autoantibodies to GPVI were found in the patient's serum using monoclonal antibody immobilization of platelet antigen (MAIPA). Sequencing of coding regions of the GPVI gene failed to show abnormalities, and mRNA for GPVI was present in the patient's platelets, pointing to a probable acquired defect in GPVI expression. Our results may provide a molecular explanation for the subgroup of patients with severely deficient collagen-induced platelet aggregation as previously described for GPS in the literature.