Glycoprotein IIb-IIIa Complex Research Articles

Ön mediastende hematomu olan ve Faktör V Leiden mutasyonu bulunan Glanzman Trombastenili bir olgu sunumu

Glanzmann thrombasthenia (GT) is an autosomal recessive disease associated with platelet aggregation dysfunction with normal platelet count, causing bleeding diathesis. GT is characterized by decreased levels or decreased function of the glycoprotein IIb-IIIa (GPIIb-IIIa) complex and therefore results in an increased tendency to bleeding as the aggregation phase of primary hemostasis cannot be sufficiently formed. Factor V Leiden mutation is a genetic mutation predisposing to thrombophilia. We present this case in which Factor V and Glanzmann thrombasthenia with hematoma in the anterior mediastinum are rarely seen together.

Functional platelet defects in children with severe chronic ITP as tested with 2 novel assays applicable for low platelet counts

AbstractImmune thrombocytopenia (ITP) is an autoimmune disease with a complex heterogeneous pathogenesis and a bleeding phenotype that is not necessarily correlated to platelet count. In this study, the platelet function was assessed in a well-defined cohort of 33 pediatric chronic ITP patients. Because regular platelet function test cannot be performed in patients with low platelet counts, 2 new assays were developed to determine platelet function: first, the microaggregation test, measuring in platelets isolated from 10 mL of whole blood the platelet potential to form microaggregates in response to an agonist; second, the platelet reactivity assay, measuring platelet reactivity to adenosine diphosphate (ADP), convulxin (CVX), and thrombin receptor activator peptide in only 150 μL of unprocessed whole blood. Patients with a severe bleeding phenotype demonstrated a decreased aggregation potential upon phorbol myristate acetate stimulation, decreased platelet degranulation following ADP stimulation, and a higher concentration of ADP and CVX needed to activate the glycoprotein IIbIIIa complex compared with patients with a mild bleeding phenotype. In conclusion, here we have established 2 functional tests that allow for evaluation of platelet function in patients with extremely low platelet counts (<109). These tests show that platelet function is related to bleeding phenotype in chronic ITP.

Pharmacogenetics of the Antiplatelet Effect of Aspirin

The concept of "pharmacogenetics" addresses genetically determined variation in how individuals respond to drugs. Accordingly, specific genetic variants have been suggested as contributors to a reduced response to various antiplatelet drugs. Aspirin is a cornerstone in secondary cardiovascular prevention and has been thoroughly investigated. The efficacy of aspirin is well documented, although with considerable interindividual variation. According to meta-analyses, a reduced antiplatelet effect of aspirin confers an increased risk of cardiovascular events. The platelet response to aspirin is assessed by in vitro evaluation of thromboxane-dependent platelet function. A reduced antiplatelet effect of aspirin can be explained by several mechanisms, which are largely determined by clinical, pharmacodynamic, biological and genetic factors. During the past decade, numerous studies have identified genetic polymorphisms significantly associated with cardiovascular events and modulating the antiplatelet effect of aspirin. However, results have been contradictory allowing only few firm conclusions to be drawn. Polymorphisms in genes encoding glycoproteins (IIb/IIIa, Ia/IIa, VI and Ibα), cyclooxygenases (1 and 2), adenosine diphosphate receptors (P2Y1 and P2Y12) and proteins of importance for haemostasis (thromboxane A2 receptor, coagulation factor XIII, etc.) have been investigated. In particular, a polymorphism in the gene encoding glycoprotein IIb/IIIa has been associated with a reduced antiplatelet effect of aspirin. The additive value of an individual's genetic makeup in predicting the antiplatelet effect of aspirin and the risk of cardiovascular events remains controversial. The present review outlines the pharmacology of aspirin and provides an overview of specific genetic variations considered to influence the antiplatelet effect of aspirin.

Antiaggregatory effect of midazolam on human platelets during monitored anesthesia care for trans-vaginal oocyte retrieval

BackgroundMidazolam plays a major role in sedation and large doses might be used under certain circumstances. Trans-vaginal oocyte retrieval is one of the most painful procedures in which monitored anesthesia care (MAC) is needed. The mechanism by which midazolam exerts anti-aggregation of human platelets is still unclear.This work aimed to investigate the in vivo suppressing effect of midazolam on human platelet aggregation.MethodsSixty adult females ASA I–II, scheduled for trans-vaginal ultrasound oocyte retrieval by combined intravenous midazolam sedation with paracervical local nerve block were included. At the end of the procedure, it was found that the range of total midazolam consumption was 4.9–9 mg. Thereby, patients were divided into three groups according to the total consumption of midazolam: Group I (4.9–6.2 mg), Group II (6.3–7.6 mg), Group III (7.7–9 mg). Patients who needed other drugs for sedation were excluded. Pre- and post-operative Bleeding Time (BT) were recorded. Two blood samples were collected from each patient to test the inhibitory effect of midazolam on platelet aggregation. Platelet aggregation was tested using platelet-aggregometer. ELISA was used to measure thromboxane B2 formation. Flowcytometry was used to evaluate whether glycoprotein IIb–IIIa complex is the receptor site for the antiplatelet action of midazolam.ResultsOnly 51 patients completed the study: Group I (n = 16), Group II (n = 24), and Group III (n = 11). Bleeding Time (BT) showed significant prolongation in group III compared to basal levels. Midazolam suppressed platelet aggregation in a dose-dependent manner as detected by aggregometer. Glycoprotein IIb–IIIa complex is not its site of action as shown by flowcytometric analysis. Lastly, thromboxane B2 was significantly inhibited by midazolam.ConclusionThis study revealed that midazolam dose-dependently inhibits platelet aggregation by a mechanism not involving the binding of glycoprotein IIb–IIIa complex (fibrinogen receptor). It was also found that midazolam inhibits thromboxane A2 formation.

Glanzmann thrombasthenia in a 17‐year‐old Peruvian Paso mare

A 17-year-old Peruvian Paso mare was evaluated for bilateral epistaxis that had been present for at least 3 years. The mare had mild anemia, platelet count within the reference interval, unremarkable coagulation times, and a negative Coggins test. On endoscopic examination, structural abnormalities were not observed in the nasal cavities, pharynx, larynx, trachea, or either guttural pouch, but petechiation was noted in the nasal mucosa. Additional tests revealed prolonged cutaneous bleeding time, normal concentration of von Willebrand factor antigen, an abnormal clot retraction test, and failure of plalelet aggregation in response to agonists, suggesting a functional disorder of platelets. Genetic analysis indicated the horse was homozygous for a 10-base-pair deletion that included the last 3 base pairs of exon 11 and the first 7 base pairs of intron 11 of the gene encoding glycoprotein IIb. The diagnosis was Glanzmann thrombasthenia (GT) caused by a structural defect in glycoprotein IIb. GT is an autosomal recessive disorder caused by a defect in the glycoprotein IIb-IIIa complex on platelet surfaces. Separate genes encode each glycoprotein, and mutations in either gene can result in GT. This case of GT is unique given the age of the mare at the time of diagnosis. We conclude that GT, although an inherited disorder, should be considered in horses with suspected dysfunctional platelets, regardless of age.

Predictive Value of Platelet Activation for the Rate of Cognitive Decline in Alzheimer's Disease Patients

Vascular risk factors contribute to the progression of dementia in Alzheimer's disease (AD) and influence platelet activation. However, the degree of platelet activation as a possible underlying mechanism of this progression has not been studied till now. Significantly higher baseline expression of both platelet activation biomarkers, activated glycoprotein IIb-IIIa complex and P-selectin, was observed in patients with AD with fast cognitive decline compared with AD patients with slow cognitive decline during a 1-year follow-up period. These results suggest that platelet activation could be a putative prognostic biomarker for the rate of cognitive decline and a potential new treatment target in AD patients.

Cross-reaction of antibody against Helicobacter pylori urease B with platelet glycoprotein IIIa and its significance in the pathogenesis of immune thrombocytopenic purpura

Many clinical investigations have suggested that Helicobacter pylori (H. pylori) infection might be associated with immune thrombocytopenic purpura (ITP), but its role in the pathogenesis of ITP is unsettled. In this study, we cultured H. pylori, produced recombinant H. pylori urease (ure) B, and then prepared monoclonal antibody (MoAb) against ureB, 1F11, both 1F11 and MoAb against human platelet glycoprotein (GP) IIIa, SZ21, could bind to the band of GP IIIa of normal platelet lysate, but not to that from a patient with Glanzmann thrombasthenia (GT) whose GP IIb-IIIa complex was absent. Flow cytometry showed that normal platelets were reacted with 1F11 and SZ21, while GT platelets were not. In immuno-radiometric assay, the binding of (125)I-labeled 1F11 to GP IIIa was higher than that to GP Ib, GP IIb, GP VI, and P-selectin. 1F11 could partly compete with SZ21 in a binding to platelet surface. In addition, 1F11 inhibited platelet aggregation induced by adenosine diphosphate, but had no effect on platelet P-selectin expression or Thromboxane B(2) production of platelets. These results indicate that H. pylori ureB antibody could cross-react with human platelet GP IIIa and partly inhibit platelet aggregation. UreB may be a crucial component of H. pylori involved in the pathogenesis of a subset of ITP.

Periodontitis is associated with platelet activation

There is an epidemiological association between periodontitis and cardiovascular disease (CVD). In periodontitis, low grade systemic inflammation and bacteremia occur regularly. Such events may contribute to platelet activation and subsequent pro-coagulant state. This study aimed to investigate platelet activation in periodontitis patients. The study is composed of two parts. In the first part, plasma levels of soluble(s) P-selectin and sCD40 ligand were measured as general markers of platelet activation in periodontitis patients ( n = 85) and in healthy controls ( n = 35). In the second part, surface-exposed P-selectin and the ligand-binding conformation of the glycoprotein IIb–IIIa complex (binding of PAC-1 antibody) were determined on individual platelets in whole blood of periodontitis patients ( n = 18) and controls ( n = 16). Patients had significantly elevated plasma levels of sP-selectin ( P < 0.001) and increased binding of PAC-1 on isolated platelets ( P = 0.033). Platelet activation was more pronounced in the patients with more severe periodontal disease, showing a severity-dependence. The levels of sCD40 ligand and of platelet-bound P-selectin were not increased. Periodontitis is associated with increased platelet activation. Since platelet activation contributes to a pro-coagulant state and constitutes a risk for atherothrombosis, platelet activation in periodontitis may partly explain the epidemiological association between periodontitis and CVD.

Periodontal considerations for Glanzmann′s thrombasthenic patient

Glanzmann's thrombasthenia (GT) was reported and described as a bleeding diathesis seen in children and characterized by diminished clot retraction. The disorder is caused by a deficiency in the platelet membrane glycoprotein IIb–IIIa complex, with bleeding due to defective platelet hemostatic plug formation. The recurrent features of GT include purpura, epistaxis, gingival hemorrhage, and menorrhagia. GT being an autosomal recessive trait is reported to be especially prevalent in populations where intermarriage is common. Typically, the patients are diagnosed in infancy within the age of five. Though no differences appear to occur based on sex men more frequently present with gingival bleeding. We report the case of a female patient with GT who presented with the chief complaint of gingival bleeding. The patient was given periodontal treatment under platelet transfusion followed by proper oral hygiene instructions. The report discusses periodontal consideration for GT patients.

P.152 Acupuncture treatment of purulent-inflammatory processes in maxillofacial area

Preincubation of human platelets with two pairs of immunologically identical monoclonal antibodies (mab) directed against epitopes on the membrane glycoprotein IIb-IIIa complex - two of them (Gi3 and Gi5) precipitating glycoproteins IIb-IIIa, and two (Gi4 and Gi6) recognizing glycoprotein IIIa -induced slightly different inhibitory effects on platelet function. Gi3 and Gi5 blocked platelet aggregation and 14C-serotonin release from platelets induced by ADP, collagen, thrombin, arachidonic acid, ionophore A 23187, platelet activating factor and ristocetin (in the presence of divalent ions). Ristocetin-induced platelet agglutination (in the presence of EDTA) was not inhibited by these mab. Gi4 had no effect on platelet aggregation and release. Gi6 had the same effect on platelet function as Gi3 and Gi5, but only when incubated with Zw(a) (PIA1)-positive platelets with the exception that collagen-induced aggregation was not affected. We conclude that the similar, though not identical, effects on platelet function induced by mab against different epitopes on the glycoprotein complex IIb-IIIa indicate an “unspecific” interference with platelet membrane function possibly due to steric hindrance by mab of the fibrinogen receptor or by inhibition of conformational changes in the GP IIb-IIIa complex necessary for exposure of the fibrinogen binding site.

Characterization of the cDNA Encoding αIIb and β3 in Normal Horses and Two Horses with Glanzmann Thrombasthenia

Glanzmann thrombasthenia (GT) is an inherited, intrinsic platelet defect characterized by a quantitative or qualitative change in the platelet glycoprotein complex IIb-IIIa (integrin alpha(IIb)beta3). The subunits are encoded by separate genes and both subunits must be expressed for a stable complex to form on the platelet surface; therefore, a defect in either gene can result in GT.

Platelet activation status decreases after menopause

Objective. The aim of the study was to investigate the impact of the climacterium (before and after menopause) on platelet activation.Background. Platelet activation has been associated to the risk of cardiovascular disease. There is much speculation about the relationship between platelet function and sex steroids, due to peculiarities of platelet action between the genders, including concerns about the influence of low estradiol status in menopausal women.Methods. By means of a cross-sectional study design, 37 female patients divided into two groups were compared. Group A consisted of ten women, mean age 43.9 years, in the premenopausal period, with normal estrogen levels; and Group B comprised 27 patients, mean age 53.0 years, who had all reached menopause. Platelet activation markers, namely P-selectin and glycoprotein IIb–IIIa complex (GPIIb–IIIa), were evaluated by flow cytometry with monoclonal antibodies. A binding index was calculated for both parameters (percentage of positive platelets × mean fluorescence of positive platelets). Also, thromboxane A2 was quantified by means of its main plasma metabolite, thromboxane B2, by enzyme immunoassay.Results. P-selectin and GPIIb–IIIa expression results revealed lower platelet activation status after menopause, as there was a decrease in both the percentage of P-selectin + platelets and of GPIIb–IIIa mean fluorescence of positive platelets, lowering both binding indices. P-selectin binding index differed significantly between Group A (12.3 ± 3, n = 10) and Group B (6.2 ± 2.9, n = 27; mean ± standard deviation (SD), p < 0.001). GPIIb–IIIa binding index also differed significantly between both groups (Group A: 18.8 ± 2.3, n = 10 vs. Group B: 16.2 ± 3.1, n = 27; mean ± SD, p < 0.0018). Plasma concentration of thromboxane B2 was 1.07 ± 0.5 pg/well before menopause (Group A, n = 10) and 1.9 ± 4.1 pg/well after menopause (Group B, n = 27), not significantly different (mean ± SD, baseline × therapy, p = 0.85).Conclusions. After the menopause, climacteric women – whose estradiol status is low – have a decreased activation platelet status compared with premenopausal women. Nevertheless, further studies on a larger sample are necessary for conclusive data regarding cardiovascular disease.

Simultaneous isolation of platelet factor 4 and glycoprotein IIb–IIIa complex from rabbit platelets, and characterization of specific chicken antibodies to assay them

Rabbits are frequently used as models for studying coagulation and platelet disorders. However, few reports on literature have dealt with the purification and characterization of rabbit platelet proteins. Herein a protocol for the simultaneous purification of rabbit platelet factor 4 (PF4) and platelet glycoprotein IIb–IIIa (GPIIb–IIIa, integrin α IIbβ 3) is described. Specific antibodies were raised in laying chicken, which were used for assaying PF4 by ELISA, and GPIIb–IIIa by direct immunofluorescence and flow cytometry. Furthermore, the binding of monoclonal antibodies specific for GPIIb–IIIa complex (P2), ligand-induced binding site of GPIIIa (LIBS1) and rabbit P-selectin (12A7), as well as of polyclonal IgY specific for rabbit GPIIb–IIIa, was compared in quiescent and thrombin-activated platelets. Polyclonal anti-rabbit PF4 IgY was a specific and sensitive probe that could be used for assaying PF4 in plasma samples. GPIIb–IIIa expression was increased in thrombin-activated platelets, as evaluated by flow cytometric analysis using P2 and polyclonal antibodies raised in chickens. Rabbit GPIIb–IIIa also exhibited a conformational modification that caused the appearance of ligand-induced binding sites. Increased P-selectin expression, used as a positive control, was also noticeable in thrombin-activated platelets. These data evidence that antibodies raised in laying chickens specific to rabbit PF4 and GPIIb–IIIa, as well as certain monoclonal antibodies specific for human GPIIb–IIIa, may be used for investigating rabbit platelet physiology.

Disruption of the long-range GPIIIa Cys5-Cys435 disulfide bond results in the production of constitutively active GPIIb-IIIa (αIIbβ3) integrin complexes

AbstractThe major platelet integrin αIIbβ3, also known as the platelet glycoprotein (GP) IIb-IIIa complex, mediates platelet aggregation by serving as the receptor for fibrinogen and von Willebrand factor. In addition to its physiologic role, GPIIb-IIIa also bears a number of clinically important alloantigenic determinants. Previous studies have shown that disruption of the long-range Cys5-Cys435 disulfide bond of the β3 subunit results in the production of isoforms that bind some, but not all, anti-PlA1 alloantibodies, suggesting that mutations in this so-called long-range disulfide bond can alter the conformation of GPIIIa. The purpose of this study was to examine the effects of either the Cys5Ala or Cys435Ala substitution of GPIIIa on the adhesive properties of the GPIIb-IIIa complex. We found that both Ala5GPIIIa and Ala435GPIIIa were capable of associating with GPIIb and were expressed normally on the cell surface when cotransfected into Chinese hamster ovary (CHO) cells. CHO cells expressing GPIIb-Ala5GPIIIa or GPIIb-Ala435IIIa bound well-characterized, conformationally sensitive ligand-induced binding site (LIBS) antibodies, and were capable of constitutively binding the fibrinogen-mimetic monoclonal antibodies Pl-55 and PAC-1, as well as soluble fibrinogen. Both GPIIb-Ala5IIIa– and GPIIb-Ala435IIIa–transfected CHO cells also bound more avidly to immobilized fibrinogen and were capable of mediating the tyrosine phosphorylation of pp125FAK on cell adhesion. These data are consistent with the notion that these regions of GPIIIa participate in the conformational change associated with receptor activation. Additionally, these studies may provide a molecular explanation for the previously reported ability of mild reducing agents to activate the GPIIb-IIIa complex and promote platelet aggregation.

Mechanisms involved in the antiplatelet activity of midazolam in human platelets.

Midazolam is widely used as a sedative and anesthetic induction agent. The aim of this study was to systematically examine the inhibitory mechanisms of midazolam in platelet aggregation. The inhibitory mechanisms of midazolam in platelet aggregation were explored by means of analysis of the platelet glycoprotein IIb-IIIa complex, phosphoinositide breakdown, intracellular Ca+2 mobilization, measurement of membrane fluidity, thromboxane B2 formation, and protein kinase C activity. In this study, midazolam dose-dependently (6-26 microm) inhibited platelet aggregation in human platelets stimulated by agonists. Midazolam also dose-dependently inhibited phosphoinositide breakdown and intracellular Ca+2 mobilization in human platelets stimulated by collagen. Midazolam (6-26 mum) significantly inhibited thromboxane A2 formation stimulated by collagen in human platelets. Moreover, midazolam (15 and 26 mum) dose-dependently decreased the fluorescence of platelet membranes tagged with diphenylhexatriene. Rapid phosphorylation of a platelet protein of Mr 47,000 (P47), a marker of protein kinase C activation, was triggered by collagen (2 microg/ml). This phosphorylation was markedly inhibited by midazolam (26 microm). These results indicate that the antiplatelet activity of midazolam may be involved in the following pathways: the effects of midazolam may initially be caused by induction of conformational changes in platelet membrane, leading to a change in the activity of phospholipase C, and subsequent inhibition of phosphoinositide breakdown and thromboxane A2 formation, thereby leading to inhibition of both intracellular Ca+2 mobilization and phosphorylation of P47 protein.

Effects of fibrinogen receptor antagonist GR144053F and aurintricarboxylic acid on platelet activation and degranulation

Activated blood platelets play crucial role in restenosis due to their fundamental significance in thrombus formation. Therefore, platelets are attractive targets for the inhibition with a variety of antagonists. In this study, we present direct evidence that GR144053F [non-peptide antagonist of glycoprotein IIb-IIIa complex (GPIIb-IIIa)] inhibits activation and degranulation of human platelets, and opposes the action of aurintricarboxylic acid (ATA), the antagonist of von Willebrand factor, which augments platelet secretion. The effects of both drugs on platelet function were monitored by using various instrumental methods. Platelet-rich plasma and whole-blood aggregation was measured by using ADP and collagen as agonists. Platelet degranulation was assessed based on the expression of surface membrane activation markers: P-selectin, glycoprotein Ib, and activated GPIIb-IIIa complex. Measurements of closure time with platelet function analyzer PFA-100™ enabled us to reason on primary hemostatic capacity and reflected both aggregability and adhesiveness. GR144053F markedly reduced primary hemostatic platelet response ( ic 50 = 114.0 ± 9.6 nM) under conditions that closely mimicked natural blood flow in circulation, and inhibited aggregation in platelet-rich plasma ( ic 50 = 17.7 ± 7.0 nM). It was equally potent inhibitor of platelet activation, degranulation, fibrinogen binding, platelet consumption, and aggregate formation. Also, ATA was efficient in inhibition of platelet aggregation and adhesion (by up to 50% at 100 μM), but the combined action of both drugs on primary haemostatic capacity was not additive. GR144053F suppressed the activating effects of ATA on platelet degranulation and secretion. Overall, our data indicate that GR144053F is not only the efficient blocker of fibrinogen binding to GPIIb-IIIa, but also hampers platelet degranulation and may attenuate the activating effects of ATA.

Clinical, Biochemical, and Molecular Aspects of Glanzmann's Thrombasthenia in Humans and Dogs

Glanzmann's thrombasthenia (GT) is an inherited, intrinsic platelet function defect that involves the platelet glycoprotein complex IIb-IIIa, also known as the fibrinogen receptor and the integrin alphaIIbbeta3. The defect was originally described by Dr. Glanzmann in humans in 1918 as a bleeding disorder that differed clinically from other known coagulopathies. Over the decades that followed, researchers determined the biochemical and molecular basis for the disease in humans. Otterhounds with thrombasthenic thrombopathia, described in the 1960s, were the only animal model that closely resembled the disease described in humans until 1996. At that time, a Great Pyrenees dog was identified with unequivocal clinical and biochemical features of Type I GT The cDNA encoding for glycoproteins IIb and IIIa were sequenced in normal dogs in 1999, allowing for identification of specific mutations causing Type I GT in both Otterhounds and Great Pyrenees dogs. Knowing the molecular basis for Type I GT in dogs as well as the cDNA sequences in normal dogs should enhance the understanding of structure/function relationships of the alphaIIbbeta3 integrin and provide an excellent animal model for studies aimed at correction of GT in humans. The following review focuses on the structure and function of this platelet receptor and reviews the molecular, biochemical, and clinical aspects of Glanzmann's thrombasthenia in humans and dogs.

A novel (288delC) mutation in exon 2 of GPIIb associated with type I Glanzmann's thrombasthenia.

This work reports the molecular genetic analysis of two patients who suffer mucocutaneous haemorrhages, prolonged bleeding time and failure of platelets to aggregate, either spontaneously or in response to agonists. The absence of platelet surface glycoprotein (GP)IIb-IIIa complexes confirmed the clinical diagnosis of Glanzmann's thrombasthenia (GT). Polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) analysis of exon 2 of GPIIb showed polymorphic bands caused by the homozygous deletion of a cytosine at position 288 relative to the translation start site. causing a shifting of the reading frame and appearance of a premature termination codon. The heterozygous relatives showed a reduced platelet content of GPIIb-IIIa, and a correlation was found between the levels of GPIIb mRNA and surface expression of GPIIb-IIIa complexes. Unlike other mRNAs carrying a nonsense mutation, (288Cdel)GPIIb does not force alternative splicing of GPIIb mRNA. As expected, co-transfection of Chinese hamster ovary (CHO) cells with cDNAs encoding GPIIIa and (288delC)GPIIb failed to enhance the surface exposure of GPIIIa. It is concluded that the (288delC)GPIIb mutation is responsible for the thrombasthenic phenotype of the patients. In addition, it has also been determined that heterodimerization of GPIIb-IIIa requires the integrity of exons 2 and 3 of GPIIb.

Platelet glycoprotein IIb/IIIa receptors and Glanzmann's thrombasthenia.

Platelet aggregation and fibrin formation are essential for the maintenance of normal hemostasis, a system designed to act quickly and effectively to arrest hemorrhage. This system is also triggered by pathogenic events, such as the rupture of an atherosclerotic plaque, which can lead to thrombotic vaso-occlusion, ischemia, and infarction. Platelets are a major contributor to these damaging and life-threatening thrombotic phenomena because of their adhesive properties, which result in the release of soluble mediators, platelet aggregation, and enhancement of thrombin generation.1 The platelet glycoprotein IIb/IIIa (GPIIb/IIIa) receptor is a key component in the pathway to platelet aggregation; consequently, this receptor has become the target for therapeutic intervention. A paradigm of this antiplatelet treatment modality is found naturally in the inherited disorder Glanzmann’s thrombasthenia. A key feature of this disease is that patients present with mucocutaneous bleeding but only rarely demonstrate spontaneous central nervous system hemorrhage,2 a feared complication of anticoagulant and antiplatelet therapy. All of the mutations that have been identified in patients with Glanzmann’s thrombasthenia result in a functional deficiency of GPIIb/IIIa receptors,2 3 and a hallmark of this disease is the absence of agonist-induced platelet aggregation. The molecular characterization of mutations causing Glanzmann’s thrombasthenia has provided a wealth of information on structure-function relations of the GPIIb/IIIa receptor. This review will briefly summarize those mutations that affect ligand-binding domains and receptor activation and present them in the context of predicted structures. More comprehensive coverage can be found in reviews discussing the structure and function of the GPIIb/IIIa receptor complex4 5 and the clinical and molecular basis of Glanzmann’s thrombasthenia.2 3 Platelets are the first line of defense in preventing blood loss from injured blood vessels via recognition and adhesion to components of the subendothelial matrix. This event is followed by formation of a platelet …

Congenital disorders of platelet signal transduction.

After injury to the blood vessel, platelets adhere to the exposed subendothelium by a process (adhesion) that involves the interaction of a plasma protein, von Willebrand factor (vWF), and a specific protein on the platelet surface, glycoprotein Ib (GPIb; the Figure⇓). Adhesion is followed by recruitment of additional platelets that form clumps, a process called aggregation (cohesion). This involves binding of fibrinogen to specific platelet surface receptors—a complex comprising glycoproteins IIb-IIIa (GPIIb-IIIa). Activated platelets release the contents of their granules (secretion or release reaction), such as ADP and serotonin from dense granules, which subsequently cause recruitment of additional platelets. In addition, platelets play a major role in coagulation mechanisms; several key enzymatic reactions occur on the platelet membrane–lipoprotein surface. A number of physiological agonists interact with specific receptors on the platelet surface to induce responses, including a change in platelet shape from discoid to spherical, aggregation, secretion, and thromboxane A2 (TxA2) production. Other agonists such as prostacyclin inhibit these responses. Ligation of the platelet receptors initiates the production or release of several intracellular messenger molecules, including Ca2+ ions, products of phosphoinositide (PI) hydrolysis by phospholipase C (PLC; diacylglycerol [DG] and inositol 1,4,5-triphosphate [InsP3]), TxA2, and cyclic nucleotides (cAMP; the Figure⇓). These subsequently induce or modulate the various platelet responses of Ca2+ mobilization, protein phosphorylation, aggregation, secretion, and liberation of arachidonic acid. The interaction between the agonist receptors and the key intracellular effector enzymes (eg, PLA2, PLC, adenylyl cyclase) is mediated by a group of GTP-binding proteins that are modulated by GTP. As in most secretory cells, platelet activation results in …