A new variant of Glanzmann's thrombasthenia (Strasbourg I). Platelets with functionally defective glycoprotein IIb-IIIa complexes and a glycoprotein IIIa 214Arg----214Trp mutation

Profiling the Genetic and Molecular Characteristics of Glanzmann Thrombasthenia: Can It Guide Current and Future Therapies?

Glanzmann thrombasthenia (GT) is the most widely studied inherited disease of platelet function. Platelets fail to aggregate due to a defect in platelet-to-platelet attachment. The hemostatic plug fails to form and a moderate to severe bleeding diathesis results. Classically of autosomal recessive inheritance, GT is caused by defects within the ITGA2B and ITGB3 genes that encode the αIIbβ3 integrin expressed at high density on the platelet surface and also in intracellular pools. Activated αIIbβ3 acts as a receptor for fibrinogen and other adhesive proteins that hold platelets together in a thrombus. Over 50 years of careful clinical and biological investigation have provided important advances that have improved not only the quality of life of the patients but which have also contributed to an understanding of how αIIbβ3 functions. Despite major improvements in our knowledge of GT and its genetic causes, extensive biological and clinical variability with respect to the severity and intensity of bleeding remains poorly understood. I now scan the repertoire of ITGA2B and ITGB3 gene defects and highlight the wide genetic and biological heterogeneity within the type II and variant subgroups especially with regard to bleeding, clot retraction, the internal platelet Fg storage pool and the nature of the mutations causing the disease. I underline the continued importance of gene profiling and biological studies and emphasize the multifactorial etiology of the clinical expression of the disease. This is done in a manner to provide guidelines for future studies and future treatments of a disease that has not only aided research on rare diseases but also contributed to advances in antithrombotic therapy.

A novel heterozygous mutation flanking the fourth calcium-binding domain of the ITGA2B gene induces severe bleeding complications: a case report and literature review

Glanzmann thrombasthenia is a rare autosomal recessive genetic disease characterized by platelet aggregation dysfunction caused by a congenital defect of platelet membrane glycoprotein IIb/IIIa (integrin αIIbβ3). Integrin αIIbβ3, a calcium-dependent heterodimer, plays a critical role in platelet aggregation. We described a boy who was hospitalized with serious epistaxis at 10 months of age who had a history of repeated petechiae and spontaneous epistaxis since birth. Flow cytometry showed normal surface expression of platelet antigens. Genetic analysis and sequencing revealed the novel missense mutation c.G1252>T (p.Gly418Cys) in ITGA2B. This heterozygous amino acid mutation flanked the fourth calcium-binding domain and may interfere with integrin biogenesis via mechanisms other than merely altering cell surface expression. We discuss the heterogeneity of the genotype and phenotype with this atypical case and review the relevant literature on mutations adjacent to or within the calcium-binding domains in Glanzmann thrombasthenia.

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

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

Pathogenic and likely pathogenic variants in at least five genes account for approximately 3% of mild isolated nonsyndromic thrombocytopenia

BACKGROUND

Thrombocytopenia has a variety of different etiologies, both acquired and hereditary. Inherited thrombocytopenia may be associated with other symptoms (syndromic forms) or may be strictly isolated. To date, only about half of all the familial forms of thrombocytopenia have been accounted for in terms of well-defined genetic abnormalities. However, data are limited on the nature and frequency of the underlying causative genetic variants in individuals with mild isolated nonsyndromic thrombocytopenia.

STUDY DESIGN AND METHODS

Thirteen known or candidate genes for isolated thrombocytopenia were included in a gene panel analysis in which targeted next-generation sequencing was performed on 448 French blood donors with mild isolated nonsyndromic thrombocytopenia.

RESULTS

A total of 68 rare variants, including missense, splice site, frameshift, nonsense, and in-frame variants (all heterozygous) were identified in 11 of the 13 genes screened. Twenty-nine percent (N = 20) of the variants detected were absent from both the French Exome Project and gnomAD exome databases. Using stringent criteria and an unbiased approach, we classified seven predicted loss-of-function variants (three in ITGA2B and four in TUBB1) and four missense variants (one in GP1BA, two in ITGB3 and one in ACTN1) as being pathogenic or likely pathogenic. Altogether, they were found in 13 members (approx. 3%) of our studied cohort.

CONCLUSION

We present the results of gene panel sequencing of known and candidate thrombocytopenia genes in mild isolated nonsyndromic thrombocytopenia. Pathogenic and likely pathogenic variants in five known thrombocytopenia genes were identified, accounting for approximately 3% of individuals with the condition.

Long-term treatment with thalidomide for severe recurrent hemorrhage from intestinal angiodysplasia in Glanzmann Thrombasthenia

Gastrointestinal angiodysplasia (GIA) is the most common cause of occult gastrointestinal bleeding (GIB) requiring often hospitalization and transfusions, especially in patients with hemorrhagic disorders. Thalidomide, impairing neo-angiogenesis, has been successfully used in the management of bleeding in patients with GIA and in particular in patients with inherited bleeding disorders. Only one case of short-term treatment with thalidomide in a patient with Glanzmann thrombasthenia (GT) and recurrent GIB due to GIA has been reported so far.We report the case of a woman with GT developing high frequency recurrent GIB due to GIA requiring repeated blood and platelet transfusions, who was treated with thalidomide obtaining a striking and stable reduction of GIB and of the requirement of platelet and blood transfusions for over 5 years. Moreover, we raise the suspicion that the association between GT and GIA may not be fortuitous.

Cryo-Electron Microscopy Structure of the αIIbβ3-Abciximab Complex

OBJECTIVE

The αIIbβ3 antagonist antiplatelet drug abciximab is the chimeric antigen-binding fragment comprising the variable regions of murine monoclonal antibody 7E3 and the constant domains of human IgG1 and light chain κ. Previous mutagenesis studies suggested that abciximab binds to the β3 C177-C184 specificity-determining loop (SDL) and Trp129 on the adjacent β1-α1 helix. These studies could not, however, assess whether 7E3 or abciximab prevents fibrinogen binding by steric interference, disruption of either the αIIbβ3-binding pocket for fibrinogen or the β3 SDL (which is not part of the binding pocket but affects fibrinogen binding), or some combination of these effects. To address this gap, we used cryo-electron microscopy to determine the structure of the αIIbβ3-abciximab complex at 2.8 Å resolution. Approach and Results: The interacting surface of abciximab is comprised of residues from all 3 complementarity-determining regions of both the light and heavy chains, with high representation of aromatic residues. Binding is primarily to the β3 SDL and neighboring residues, the β1-α1 helix, and β3 residues Ser211, Val212 and Met335. Unexpectedly, the structure also indicated several interactions with αIIb. As judged by the cryo-electron microscopy model, molecular-dynamics simulations, and mutagenesis, the binding of abciximab does not appear to rely on the interaction with the αIIb residues and does not result in disruption of the fibrinogen-binding pocket; it does, however, compress and reduce the flexibility of the SDL.

CONCLUSIONS

We deduce that abciximab prevents ligand binding by steric interference, with a potential contribution via displacement of the SDL and limitation of the flexibility of the SDL residues.

Activation mechanisms of αVβ3 integrin by binding to fibronectin: A computational study

Integrin αVβ3 plays an important role in regulating cellular activities and in human diseases. Although the structure of αVβ3 has been studied by crystallography and electron microscopy, the detailed activation mechanism of integrin αVβ3 induced by fibronectin remains unclear. In this study, we investigated the conformational and dynamical motion changes of Mn2+ -bound integrin αVβ3 by binding to fibronectin with molecular dynamics simulations. Results showed that fibronectin binding to integrin αVβ3 caused the changes of the conformational flexibility of αVβ3 domains, the essential mode of motion for the domains of αV subunit and β3 subunit and the degrees of correlated motion of residues between the domains of αV subunit and β3 subunit of integrin αVβ3. The angle of Propeller domain with respect to the Calf-2 domain of αV subunit and the angle of Hybrid domain with respect to βA domain of β3 subunit significantly increased when integrin αVβ3 was bound to fibronectin. These changes could result in the conformational change tendency of αVβ3 from a bend conformation to an extended conformation and lead to the open swing of Hybrid domain relative to βA domain of β3 subunit, which have demonstrated their importance for αVβ3 activation. Fibronectin binding to integrin αVβ3 significantly decreased the relative position of α1 helix to βA domain and that to metal ion-dependent adhesion site, stabilized Mn2+ ions binding in integrin αVβ3 and changed fibronectin conformation, which are important for αVβ3 activation. Results from this study provide important molecular insight into the "outside-in" activation mechanism of integrin αVβ3 by binding to fibronectin.

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 αIIbβ3. 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 αIIbβ3 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.

Expanding the Mutation Spectrum Affecting αIIbβ3 Integrin in Glanzmann Thrombasthenia: Screening of the ITGA2B and ITGB3 Genes in a Large International Cohort

We report the largest international study on Glanzmann thrombasthenia (GT), an inherited bleeding disorder where defects of the ITGA2B and ITGB3 genes cause quantitative or qualitative defects of the αIIbβ3 integrin, a key mediator of platelet aggregation. Sequencing of the coding regions and splice sites of both genes in members of 76 affected families identified 78 genetic variants (55 novel) suspected to cause GT. Four large deletions or duplications were found by quantitative real-time PCR. Families with mutations in either gene were indistinguishable in terms of bleeding severity that varied even among siblings. Families were grouped into type I and the rarer type II or variant forms with residual αIIbβ3 expression. Variant forms helped identify genes encoding proteins mediating integrin activation. Splicing defects and stop codons were common for both ITGA2B and ITGB3 and essentially led to a reduced or absent αIIbβ3 expression; included was a heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B causing exon skipping in seven unrelated families. Molecular modeling revealed how many missense mutations induced subtle changes in αIIb and β3 domain structure across both subunits, thereby interfering with integrin maturation and/or function. Our study extends knowledge of GT and the pathophysiology of an integrin.

Different biochemical expression pattern of platelet surface glycoproteins suggests molecular diversity of Glanzmann's thrombasthenia in Iran

Glanzmann's thrombasthenia is a rare congenital bleeding disorder characterized by lack of platelet aggregation induced by most agonists. The disease is caused by different mutations in either GPIIb or GPIIIa genes that lead to a lack or dysfunction of the αIIbβ3. Western blot analysis was performed on the platelet lysates of 95 patients with Glanzmann's thrombasthenia who were referred to the Iranian Blood Transfusion and Hemophilia Clinic. Glanzmann's thrombasthenia was diagnosed based on clinical findings and were classified according to Glanzmann's Thrombasthenia Italian Team (GLATIT) protocol. The platelet glycoprotein expression pattern in Iranian patients with Glanzmann's thrombasthenia was studied and the relationship between the platelet glycoprotein expression levels and clinical symptoms were investigated. Loss or severe reduction of platelet GpIIb and GpIIIa were observed in majority of patients (78%). The remaining patients (22%) showed a relatively sharp decline to the normal amounts of the glycoproteins. None of the patients showed expression of CD41 without CD61. Statistical analysis showed no significant relationship between clinical symptoms and expression of platelet glycoproteins. Different patterns of platelet glycoproteins expression suggest that there is variety of mutations in the patients. Unlike the universal Glanzmann's thrombasthenia database, the majority of Iranian patients may suffer from a GpIIIa gene mutation. This study also confirmed a lack of correlation between clinical manifestation and GPIIb/IIIa expression in the patients.

Glanzmann thrombasthenia: a review of ITGA2B and ITGB3 defects with emphasis on variants, phenotypic variability, and mouse models

Characterized by mucocutaneous bleeding arising from a lack of platelet aggregation to physiologic stimuli, Glanzmann thrombasthenia (GT) is the archetype-inherited disorder of platelets. Transmitted by autosomal recessive inheritance, platelets in GT have quantitative or qualitative deficiencies of the fibrinogen receptor, αIIbβ3, an integrin coded by the ITGA2B and ITGB3 genes. Despite advances in our understanding of the disease, extensive phenotypic variability with respect to severity and intensity of bleeding remains poorly understood. Importantly, genetic defects of ITGB3 also potentially affect other tissues, for β3 has a wide tissue distribution when present as αvβ3 (the vitronectin receptor). We now look at the repertoire of ITGA2B and ITGB3 gene defects, reexamine the relationship between phenotype and genotype, and review integrin structure in the many variant forms. Evidence for modifications in platelet production is assessed, as is the multifactorial etiology of the clinical expression of the disease. Reports of cardiovascular disease and deep vein thrombosis, cancer, brain disease, bone disorders, and pregnancy defects in GT are discussed in the context of the results obtained for mouse models where nonhemostatic defects of β3-deficiency or nonfunction are being increasingly described.

Do cell junction protein mutations cause an airway phenotype in mice or humans?

Cell junction proteins connect epithelial cells to each other and to the basement membrane. Genetic mutations of these proteins can cause alterations in some epithelia leading to varied phenotypes such as deafness, renal disease, skin disorders, and cancer. This review examines if genetic mutations in these proteins affect the function of lung airway epithelia. We review cell junction proteins with examples of disease mutation phenotypes in humans and in mouse knockout models. We also review which of these genes are expressed in airway epithelium by microarray expression profiling and immunocytochemistry. Last, we present a comprehensive literature review to find the lung phenotype when cell junction and adhesion genes are mutated or subject to targeted deletion. We found that in murine models, targeted deletion of cell junction and adhesion genes rarely result in a lung phenotype. Moreover, mutations in these genes in humans have no obvious lung phenotype. Our research suggests that simply because a cell junction or adhesion protein is expressed in an organ does not imply that it will exhibit a drastic phenotype when mutated. One explanation is that because a functioning lung is critical to survival, redundancy in the system is expected. Therefore mutations in a single gene might be compensated by a related function of a similar gene product. Further studies in human and animal models will help us understand the overlap in the function of cell junction gene products. Finally, it is possible that the human lung phenotype is subtle and has not yet been described.

The GPIIb/IIIa (integrin alphaIIbbeta3) odyssey: a technology-driven saga of a receptor with twists, turns, and even a bend

Starting 90 years ago with a clinical description by Glanzmann of a bleeding disorder associated with a defect in platelet function, technologic advances helped investigators identify the defect as a mutation(s) in the integrin family receptor, alphaIIbbeta3, which has the capacity to bind fibrinogen (and other ligands) and support platelet-platelet interactions (aggregation). The receptor's activation state was found to be under exquisite control, with activators, inhibitors, and elaborate inside-out signaling mechanisms controlling its conformation. Structural biology has produced high-resolution images defining the ligand binding site at the atomic level. Research on alphaIIbbeta3 has been bidirectional, with basic insights resulting in improved Glanzmann thrombasthenia carrier detection and prenatal diagnosis, assays to identify single nucleotide polymorphisms responsible for alloimmune neonatal thrombocytopenia, and the development of alphaIIbbeta3 antagonists, the first rationally designed antiplatelet agents, to prevent and treat thrombotic cardiovascular disease. The future looks equally bright, with the potential for improved drugs and the application of gene therapy and stem cell biology to address the genetic abnormalities. The alphaIIbbeta3 saga serves as a paradigm of rigorous science growing out of careful clinical observations of a rare disorder yielding both important new scientific information and improved diagnosis, therapy, and prevention of other disorders.

Tyr178 of beta3 is critical for alphaIIb maturation and macromolecular ligand binding to alphaIIbbeta3

To explore the structural basis of ligand binding to alphaIIbbeta3, we conducted a site-directed mutagenesis of Y178, which is located in the ligand-specificity region (C177-C184) of the beta3 subunit. Two mutant beta3 constructs, Y178A and Y178I, were transfected into CHO cells and co-expressed with human alphaIIb subunit on the cell surface. Our results showed that the Y178A mutation affected processing and cell surface exposure of recombinant alphaIIbbeta3 receptor, abrogated the binding of PAC-1, a ligand-mimetic antibody, to alphaIIbbeta3 pre-treated with the known activator DTT. The Y178A mutation also resulted in reduced adhesion of alphaIIbbeta3 on immobilized fibrinogen. In contrast, the interaction of alphaIIbbeta3 with the small molecular ligand RGDS was unaffected by Y178A mutation, as evidenced by the elevated LIBS-1 epitope expression following RGDS addition. Interestingly however, Y178I mutation did not affect the receptor synthesis and function at all. As for post-receptor occupancy, neither Y178A nor Y178I prevented alphaIIbbeta3 translocation to focal adhesion contacts. These results suggest that Y178 is involved in alphaIIb maturation and alphaIIbbeta3 complex expression. This residue is also critical for alphaIIbbeta3 interaction with its macromolecular ligand or ligand-mimetic mAb, possibly due to its involvement in other ligand-binding sites distinct from the RGD-binding pocket. We also propose that a residue with appropriate side-chain size and hydrophobicity at position 178 is spatially required for formation of the correct tertiary structure of the site.

Glanzmann thrombasthenia

Glanzmann thrombasthenia (GT) is a rare autosomal recessive bleeding syndrome affecting the megakaryocyte lineage and characterized by lack of platelet aggregation. The molecular basis is linked to quantitative and/or qualitative abnormalities of alphaIIb beta3 integrin. This receptor mediates the binding of adhesive proteins that attach aggregating platelets and ensure thrombus formation at sites of injury in blood vessels. GT is associated with clinical variability: some patients have only minimal bruising while others have frequent, severe and potentially fatal hemorrhages. The site of bleeding in GT is clearly defined: purpura, epistaxis, gingival hemorrhage, and menorrhagia are nearly constant features; gastrointestinal bleeding and hematuria are less common. In most cases, bleeding symptoms manifest rapidly after birth, even if GT is occasionally only diagnosed in later life. Diagnosis should be suspected in patients with mucocutaneous bleeding with absent platelet aggregation in response to all physiologic stimuli, and a normal platelet count and morphology. Platelet alphaIIb beta3 deficiency or nonfunction should always be confirmed, for example by flow cytometry. In order to avoid platelet alloimmunisation, therapeutic management must include, if possible, local hemostatic procedures and/or desmopressin (DDAVP) administration. Transfusion of HLA-compatible platelet concentrates may be necessary if these measures are ineffective, or to prevent bleeding during surgery. Administration of recombinant factor VIIa is an increasingly used therapeutic alternative. GT can be a severe hemorrhagic disease, however the prognosis is excellent with careful supportive care.

Inherited Platelet Disorders

The inherited platelet disorders are a heterogeneous collection of rare diseases that are infrequently encountered in clinical practice. They are, however, fascinating abnormalities, which have taught us a great deal about normal platelet biochemistry and physiology. In this section of the presentation we will review disorders of the platelet membrane, platelet granule packaging disorders, the hereditary macrothrombocytopenias, platelet signaling disorders and disorders of platelet coagulant function. The molecular basis of the disorders, the cardinal features of their clinical presentation and best methods to make their diagnosis and the latest information regarding therapy will be presented.

Crystal Structure of Trimestatin, a Disintegrin Containing a Cell Adhesion Recognition Motif RGD

Disintegrins are a family of small proteins containing an Arg-Gly-Asp (RGD) sequence motif that binds specifically to integrin receptors. Since the integrin is known to serve as the final common pathway leading to aggregation via formation of platelet-platelet bridges, disintegrins act as fibrinogen receptor antagonists. Here, we report the first crystal structure of a disintegrin, trimestatin, found in snake venom. The structure of trimestatin at 1.7A resolution reveals that a number of turns and loops form a rigid core stabilized by six disulfide bonds. Electron densities of the RGD sequence are visible clearly at the tip of a hairpin loop, in such a manner that the Arg and Asp side-chains point in opposite directions. A docking model using the crystal structure of integrin alphaVbeta3 suggests that the Arg binds to the propeller domain, and Asp to the betaA domain. This model indicates that the C-terminal region is another potential binding site with integrin receptors. In addition to the RGD sequence, the structural evidence of a C-terminal region (Arg66, Trp67 and Asn68) important for disintegrin activity allows understanding of the high affinity and selectiveness of snake venom disintegrin for integrin receptors. The crystal structure of trimestatin should provide a useful framework for designing and developing more effective drugs for controlling platelet aggregation and anti-angiogenesis cancer.

Binding model for nonpeptide antagonists of alpha(v)beta(3) integrin

A binding model for nonpeptide antagonists of integrin alpha(v)beta(3) has been developed through docking analyses utilizing the MMFFs force field and the recently published crystal structure, 1JV2. Results of this docking study have led to the identification of a novel binding model for selective antagonists of alpha(v)beta(3) over alpha(IIb)beta(3) integrins. Four different chemical classes are shown to bind in a similar fashion providing a measure of confidence in the proposed model. All alpha(v)beta(3) and alpha(IIb)beta(3) antagonists have a basic nitrogen separated some distance from a carboxylic acid to mimic RGD. For the alpha(v)beta(3) antagonists under present consideration, these charged ends are separated by twelve bonds. The basic nitrogen of the active alpha(v)beta(3) ligands are shown to interact with D150 of alpha(v) and the ligands' carboxylic acid interact with R214 of beta(3) while adopting an extended conformation with minimal protein induced internal strain. In addition, an energetically favorable interaction is found with all of the active alpha(v)beta(3) molecules with Y178 of alpha(v) when docked to the crystallographically determined structure. This novel interaction may be characterized as pi-pi stacking for the most active of the alpha(v)beta(3) selective antagonists. The proposed model is consistent with observed activity as well as mutagenicity and photoaffinity cross-linking studies of the alpha(v)beta(3) integrin.

Dissociation between fibrinogen and fibrin interaction with platelets in patients with different subtypes of Glanzmann's thrombasthenia: studies in an ex vivo perfusion chamber model

To explore the possible role of a residual or variant alphaIIbbeta3 integrin (alphaIIbbeta3) in thrombogenesis, we used a new ex vivo perfusion chamber model to examine blood from patients with different subtypes of Glanzmann's thrombasthenia (GT). Non-anticoagulated blood was perfused through capillaries coated with type III collagen for 4.5 min (shear rate: 1600/s). Platelet deposition was quantified as platelet adhesion and mean thrombus size volume; fibrin and von Willebrand Factor (VWF) were specifically revealed by immunohistochemistry. In two patients with variant and in one patient with type II GT, platelet adhesion was maximal and we observed an unexpected formation of thrombi that were smaller than normal in size. These thrombi were surrounded by a thick meshwork that displayed a strong staining for fibrin and VWF. In two patients with heterozygous GT, platelet adhesion and thrombogenesis were normal. In two patients with type I GT, there was no thrombus formation, although platelet adhesion was also maximal. These data suggest the existence of a substitute pathway for thrombogenesis mediated by fibrin and possibly alphaIIbbeta3 (alphaIIbbeta3 at a reduced level, as in type II, and/or abnormal) as this fibrin network was not observed in type I GT with no alphaIIbbeta3. These interactions might facilitate haemostasis and even lead to thrombosis under certain favourable conditions. Furthermore, these data might have pharmacological relevance to the development of anti-alphaIIbbeta3 antithrombotic agents.

A Ser752-->Pro substitution in the cytoplasmic domain of beta3 in a Glanzmann thrombasthenia variant fails to prevent interactions between the alphaIIbbeta3 integrin and the platelet granule pool of fibrinogen

A Glanzmann thrombasthenia variant with a beta3 Ser752-->Pro cytoplasmic domain substitution has platelets that fail to aggregate or bind soluble fibrinogen (Fg) after activation. Despite this, Fg is normally present in the alpha-granules. We have used immunoelectron microscopy to examine the reactivity of Fg with the different pools of alphaIIbbeta3 in the patient's platelets. Immunogold labelling was performed on cryosections using an anti-ligand-induced binding site (LIBS) monoclonal antibody (mAb), which binds to alphaIIbbeta3 only when Fg is bound, or a mixture of two anti-receptor-induced binding site (RIBS) mAbs that specifically recognize receptor-bound Fg. Labelling of the alpha-granule membrane and channels of the surface-connected canalicular system in unstimulated platelets confirmed that the mutated alphaIIbbeta3 retains the capacity to transport Fg. When the patient's platelets were stimulated with ADP in the presence of Fg, as expected there was a much-decreased activation of surface-exposed alphaIIbbeta3. However, thrombin-induced activation was associated with both secretion and a rapid increase in the labelling of internal membrane systems by anti-RIBS and anti-LIBS mAbs, with mobilization of the internal Fg pool. Yet labelling on the surface of the patient's platelets was transient. Our studies implied that alphaIIbbeta3 in platelets may bind fibrinogen in different activation states and that this patient specifically lacked high-affinity binding.

Molecular basis of Glanzmann's Thrombasthenia and current strategies in treatment

Glanzmann Thrombasthenia, an exceptional inherited platelet disorder is characterized by a complete lack of platelet aggregation due to a defect in the alpha(IIb)beta(3) complex or to a qualitative abnormality of this complex. Advances in molecular biology have permitted to precise the molecular abnormality on alpha(IIb) or beta(3) genes responsible for the disease and have also contributed to a better knowledge of normal platelet physiology. Hemorrhages are the main clinical problem. Current principles of therapeutic management are proposed, with special reference to the risk of platelet alloimmunisation.

Identification and characterization of two cation binding sites in the integrin beta 3 subunit

The midsegment of the beta(3) subunit has been implicated in the ligand and cation binding functions of the beta(3) integrins. This region may contain a metal ion-dependent adhesion site (MIDAS) and fold into an I domain-like structure. Two recombinant fragments, beta(3)-(95-373) and beta(3)-(95-301), were expressed and found to bind fibrinogen. Whereas 0.1 mm Ca(2+) supported ligand binding to both recombinant fragments, 1.0 mm Ca(2+) suppressed binding to the longer but not the shorter fragment. These properties suggest that beta(3)-(95-373) contains both the ligand-competent (LC) and inhibitory (I) cation binding sites, and beta(3)-(95-301) lacks the I site. In equilibrium dialysis experiments, beta(3)-(95-373) contained two divalent cation binding sites, one reactive with either Mg(2+) or Ca(2+) and one Ca(2+)-specific, whereas beta(3)-(95-301) lacked the Ca(2+)-specific site. Mutant forms of beta(3)-(95-373) suggested that the LC site is a MIDAS motif involving Asp(119), Ser(121), Ser(123), Asp(217), and/or Glu(220) as coordination sites, and the I site was dependent upon residues within beta(3)-(301-323). In a molecular model of beta(3)-(95-373), a second Ca(2+) could be docked onto a flexible loop in close proximity to the MIDAS. These results indicate that the ligand competent and Ca(2+)-specific inhibitory cation binding sites are distinct and reside in beta(3)-(95-373).

Missense mutations in the beta(3) subunit have a different impact on the expression and function between alpha(IIb)beta(3) and alpha(v)beta(3)

Alpha(IIb)beta(3) and alpha(v)beta(3) belong to the beta(3) integrin subfamily. Although the beta(3) subunit is a key regulator for the biosynthesis of beta(3) integrins, it remains obscure whether missense mutations in beta(3) may induce the same defects in both alpha(IIb)beta(3) and alpha(v)beta(3). In this study, it is revealed that thrombasthenic platelets with a His280Pro mutation in beta(3), which is prevalent in Japanese patients with Glanzmann thrombasthenia, did contain significant amounts of alpha(v)beta(3) (about 50% of control) using sensitive enzyme-linked immunosorbent assay. Expression studies showed that the His280Probeta(3) mutation impaired alpha(IIb)beta(3) expression but not alpha(v)beta(3) expression in 293 cells. To extend these findings, the effects of several beta(3) missense mutations leading to an impaired alpha(IIb)beta(3) expression on alpha(v)beta(3) function as well as expression was examined: Leu117Trp, Ser162Leu, Arg216Gln, Cys374Tyr, and a newly created Arg216Gln/Leu292Ser mutation. Leu117Trp and Cys374Tyr beta(3) mutations did impair alpha(v)beta(3) expression, while Ser162Leu, Arg216Gln, and Arg216Gln/Leu292Ser mutations did not. With regard to ligand binding function, Ser162Leu mutation induced especially distinct effects between 2 beta(3) integrins: it markedly impaired ligand binding to alpha(IIb)beta(3) but not to alpha(v)beta(3) at all. These data clearly demonstrate that the biosynthesis and the ligand binding function of alpha(IIb)beta(3) and those of alpha(v)beta(3) are regulated in part by different mechanisms. Present data would be a clue to elucidate the regulatory mechanism of expression and function of beta(3) integrins.

A point mutation in the cysteine-rich domain of glycoprotein (GP) IIIa results in the expression of a GPIIb-IIIa (alphaIIbbeta3) integrin receptor locked in a high-affinity state and a Glanzmann thrombasthenia-like phenotype

This article reports a Glanzmann thrombasthenia (GT) patient, N.M., with a point mutation in the third cysteine-rich repeat of beta3-integrin or platelet glycoprotein (GP) IIIa, leading to the expression of a constitutively activated fibrinogen receptor. The diagnosis of GT was based on a severely reduced platelet-aggregation response to a series of agonists and approximately 20% of surface-expressed GPIIb-IIIa. The patient's GPIIb-IIIa constitutively expressed epitopes recognized by antibodies to ligand-induced binding sites (LIBS) and also spontaneously bound the fibrinogen-mimetic antibody, PAC-1. Furthermore, significant amounts of bound fibrinogen were detected on his platelets ex vivo. No signs of platelet activation were observed on sections of unstimulated platelets from N.M. by electron microscopy. Immunogold labeling highlighted the presence of surface-bound fibrinogen but revealed platelet heterogeneity with regard to the surface density. When the patient's platelets were stimulated by thrombin-receptor activating peptide, amounts of surface-expressed GPIIb-IIIa increased and the aggregation response improved, although it failed to normalize. Platelets from N.M. were able to adhere and spread on immobilized fibrinogen. Sequence analysis of genomic DNA from N.M. revealed a homozygous g1776T>C mutation in GPIIIa, leading to a Cys560Arg amino acid substitution. A stable Chinese hamster ovary (CHO) cell line was prepared expressing surface GPIIb-Arg560IIIa. Like platelets from the patient, GPIIb-Arg560IIIa-transfected CHO cells constitutively bound LIBS antibodies and PAC-1. They also showed an enhanced ability to adhere on surface-bound fibrinogen. Overall, these data demonstrate that a gain-of-function mutation can still be associated with a thrombasthenic phenotype even though platelets show spontaneous fibrinogen binding.

A Val193Met mutation in GPIIIa results in a GPIIb/IIIa receptor with a constitutively high affinity for a small ligand

We have identified a patient designated as (GTa) with Glanzmann's Thrombasthenia (GT) diagnosed on the basis of a prolonged bleeding time and failure of the patient's platelets to aggregate. The number of glycoprotein (GP)IIb/IIIa receptors on the platelet surface was 37% of normal and those receptors displayed a defect in soluble fibrinogen binding. Nevertheless, GTa platelets showed increased adhesion to solid-phase fibrinogen and binding affinity for the RGD-mimetic (3)H-SC52012, a non-peptide GPIIb/IIIa antagonist. Dithiothreitol (DTT) and ADP enhanced the affinity for [(3)H]-SC52012 in normal platelets, but had little effect in GTa platelets. These findings suggested that GTa platelets were locked in an altered affinity state. Genetic analysis showed that GTa was a compound heterozygote for the GPIIIa gene. One allele showed a deletion at the 3' end of exon 3 resulting in a premature stop codon. The second GPIIIa allele had a G to A transition at nucleotide 577, resulting in a Val193Met substitution. HEK 293T cells transfected with mutant GPIIb/IIIaV193M bound [(3)H]-SC52012 with a higher affinity than wild-type GPIIb/IIIa, and this was not increased by DTT. The mutant receptor distinguishes between platelet adhesion and aggregation, and demonstrates the phenotype that may be expected when platelet aggregation alone is inhibited.

Demystified...adhesion molecule deficiencies

The basic physiology of leucocyte emigration from the intravascular space into the tissues is now known to be dependent on a class of cell surface molecules that have come to be known as adhesion molecules. Many cell-cell interactions are dependent on adhesion and signal transduction via the various adhesion molecules, particularly the integrins. The study of the functions of these molecules has been enhanced by the development of blocking and activating monoclonal antibodies, knockout mice, and by the rare "experiments of nature" in the human population, in whom there is absence or dysfunction of one of the adhesion molecules. This review describes these leucocyte adhesion defects and discusses how they have provided important insights into the function of these molecules.

Expression and function of calcium binding domain chimeras of the integrins alpha(IIb) and alpha(5)

To further identify amino acid domains involved in the ligand binding specificity of alpha(IIb)beta(3), chimeras of the conserved calcium binding domains of alpha(IIb) and the alpha subunit of the fibronectin receptor alpha(5)beta(1) were constructed. Chimeras that replaced all four calcium binding domains, replaced all but the second calcium binding domain of alpha(IIb) with those of alpha(5), or deleted all four calcium binding domains were synthesized but not expressed on the cell surface. Additional chimeras exchanged subsets or all of the variant amino acids in the second calcium binding domain, a region implicated in ligand binding. Cell surface expression of each second calcium binding domain mutant complexed with beta(3) was observed. Each second calcium binding domain mutant was able to 1) bind to immobilized fibrinogen, 2) form fibrinogen-dependent aggregates after treatment with dithiothreitol, and 3) bind the activation-dependent antibody PAC1 after LIBS 6 treatment. Soluble fibrinogen binding studies suggested that there were only small changes in either the K(d) or B(max) of any mutant. We conclude that chimeras of alpha(IIb) containing the second calcium binding domain sequences of alpha(5) are capable of complexing with beta(3), that the complexes are expressed on the cell surface, and that mutant complexes are capable of binding both immobilized and soluble fibrinogen, suggesting that the second calcium binding domain does not determine ligand binding specificity.

Structural requirements of echistatin for the recognition of alpha(v)beta(3) and alpha(5)beta(1) integrins

There are key differences between the amino acid residues of the RGD loops and the C termini of echistatin, a potent antagonist of alpha(IIb)beta(3), alpha(v)beta(3) and alpha(5)beta(1), and eristostatin, a similar disintegrin selectively inhibiting alpha(IIb)beta(3). In order to identify echistatin motifs required for selective recognition of alpha(v)beta(3) and alpha(5)beta(1) integrins, we expressed recombinant echistatin, eristostatin, and 15 hybrid molecules. We tested them for their ability to inhibit adhesion of different cell lines to fibronectin and von Willebrand factor and to express ligand-induced binding site epitope. The results showed that Asp(27) and Met(28) support recognition of both alpha(v)beta(3) and alpha(5)beta(1). Replacement of Met(28) with Asn completely abolished echistatin's ability to recognize each of the integrins, while replacement of Met(28) with Leu selectively decreased echistatin's ability to recognize alpha(5)beta(1) only. Eristostatin in which C-terminal WNG sequence was substituted with HKGPAT exhibited new activity with alpha(5)beta(1), which was 10-20-fold higher than that of wild type eristostatin. A hypothesis is proposed that the C terminus of echistatin interacts with separate sites on beta(1) and beta(3) integrin molecules.

DNA sequence of the canine platelet beta3 gene from cDNA: comparison of canine and mouse beta3 to segments that encode alloantigenic sites and functional domains of beta3 in human beings

The platelet glycoprotein complex alphaIIb beta3 is required for platelet-fibrinogen binding and platelet aggregation. This study was designed to characterize the nucleotide sequence of the canine platelet beta3 gene from cDNA. The nucleotide and deduced amino acid sequences of the canine beta3 gene were 92% and 96% homologous, respectively, with the sequences previously established for the beta3 gene of human beings. Within the beta3 gene, the nucleotide sequence of cDNA prepared from canine platelets shared homology of 89% for the cytoplasmic domain, 93% for the transmembrane domain, 92% for the extracellular domain, 94% for the arginine-glycine-aspartic acid (RGD) binding domain, and 97% for the region associated with Ca2+-dependent stabilization of the alphaIIb beta3 fibrinogen-binding pocket. The deduced amino acid sequence of canine beta3 was 100%, 97%, 96%, and 95% homologous with the cytoplasmic, transmembrane, extracellular, and RGD-binding domains, respectively, and was 100% homologous with the region associated with Ca2+-dependent stabilization of the alphaIIb beta3 fibrinogen-binding pocket of beta3 in human beings. The canine platelet cDNA signal peptide segment of the beta3 gene encodes for 22 amino acids, as compared with 26 amino acids previously reported for human beings. The deduced amino acid sequence of canine beta3 corresponds to the high-frequency allelic form for five of the six alloantigenic sites reportedly associated with human platelets: Leu33Leu40Pro407Arg489Arg636. The apparent amino acid residue in position 143 (Pen alloantigen) of canine platelet beta3 is histidine compared with arginine in human beings. Knowledge of the beta3 gene nucleotide sequence of normal dogs will facilitate the understanding of platelet alphaIIb beta3 structure-function relationships.

A new model of dual interacting ligand binding sites on integrin alphaIIbbeta3

The platelet integrin alphaIIbbeta3 mediates platelet aggregation and platelet adhesion. This integrin is the key to hemostasis and also to pathologic vascular occlusion. A key domain on alphaIIbbeta3 is the ligand binding site, which can bind to plasma fibrinogen and to a number of Arg-Gly-Asp (RGD)-type ligands. However, the nature and function of the ligand binding pocket on alphaIIbbeta3 remains controversial. Some studies suggest the presence of two ligand binding pockets, whereas other reports indicate a single binding pocket. Here we use surface plasmon resonance to show that alphaIIbbeta3 contains two distinct ligand binding pockets. One site binds to fibrinogen, and a separate site binds to RGD-type ligands. More importantly, however, the two ligand binding pockets are interactive. RGD-type ligands are capable of binding to alphaIIbbeta3 even when it is already occupied by fibrinogen. Once bound, RGD-type ligands induce the dissociation of fibrinogen from alphaIIbbeta3. This allosteric cross-talk has important implications for anti-platelet therapy because it suggests a novel approach for the dissolution of existing platelet thrombi.

A Mutation in the  Subunit of the Platelet Integrin IIbβ3 Identifies a Novel Region Important for Ligand Binding

An unbiased genetic approach was used to identify a specific amino acid residue in the IIb subunit important for the ligand binding function of the integrin IIbβ. Chemically mutagenized cells were selected by flow cytometry based on their inability to bind the ligand mimetic antibody PAC1 and a cell line containing a single amino acid substitution in IIb at position 224 (D→V) was identified. Although well expressed on the surface of transfected cells, IIbD224Vβ3 as well as IIbD224Aβ3 did not bind IIbβ3-specific ligands or a RGD peptide, a ligand shared in common with vβ3. Insertion of exon 5 of IIb, residues G193-W235, into the backbone of the v subunit did not enable the chimeric receptor to bind IIbβ3-specific ligands. However, the chimeric receptor was still capable of binding to a RGD affinity matrix. IIbD224 is not well conserved among other integrin  subunits and is located in a region of significant variability. In addition, amino acid D224 lies within a predicted loop of the recently proposed β-propeller model for integrin  subunits and is adjacent to a loop containing amino acid residues previously implicated in receptor function. These data support a role for this region in ligand binding function of the IIbβ3 receptor.

A Mutation in the  Subunit of the Platelet Integrin IIbβ3 Identifies a Novel Region Important for Ligand Binding

An unbiased genetic approach was used to identify a specific amino acid residue in the IIb subunit important for the ligand binding function of the integrin IIbβ. Chemically mutagenized cells were selected by flow cytometry based on their inability to bind the ligand mimetic antibody PAC1 and a cell line containing a single amino acid substitution in IIb at position 224 (D→V) was identified. Although well expressed on the surface of transfected cells, IIbD224Vβ3 as well as IIbD224Aβ3 did not bind IIbβ3-specific ligands or a RGD peptide, a ligand shared in common with vβ3. Insertion of exon 5 of IIb, residues G193-W235, into the backbone of the v subunit did not enable the chimeric receptor to bind IIbβ3-specific ligands. However, the chimeric receptor was still capable of binding to a RGD affinity matrix. IIbD224 is not well conserved among other integrin  subunits and is located in a region of significant variability. In addition, amino acid D224 lies within a predicted loop of the recently proposed β-propeller model for integrin  subunits and is adjacent to a loop containing amino acid residues previously implicated in receptor function. These data support a role for this region in ligand binding function of the IIbβ3 receptor.

R to Q Amino Acid Substitution in the GFFKR Sequence of the Cytoplasmic Domain of the Integrin IIb Subunit in a Patient With a Glanzmann’s Thrombasthenia-Like Syndrome

The integrin IIbβ3 mediates platelet aggregation through its fibrinogen and adhesive protein-binding properties. Particular interest concerns the role of the cytoplasmic domains of IIb and β3. We now report the molecular analysis of IIbβ3 from a patient with a Glanzmann’s thrombasthenia-like syndrome for whom the principal characteristics are an approximate 50% total platelet content of IIbβ3 but with a much lower proportion in the surface pool (Hardisty et al, Blood 80:696, 1992). Polymerase chain reaction (PCR) single-strand conformational polymorphism and DNA sequencing showed a heterozygous mutation giving rise to amino acid substitution R995 to Q in the GFFKR sequence of the cytoplasmic domain of IIb. Reverse transcriptase-PCR and polymorphism analysis only detected mRNA for the mutated allele of the IIb gene and a single allele of the β3 gene in his platelets, suggesting other unidentified defects. Site-directed mutagenesis followed by transient expression of the mutated IIb together with wild-type β3 in Cos-7 cells resulted in a markedly decreased expression of the complex at the cell surface when compared with cells transfected with wild-type IIb and β3. Flow cytometry with PAC-1 and a stable Chinese hamster ovary–transfected cell line showed that the mutated receptor was not locked into a high activation state, although it became so in the presence of the activating antibody, anti-LIBS6. This is the first reported natural mutation in the highly conserved GFFKR sequence of the IIb cytoplasmic domain.

R to Q Amino Acid Substitution in the GFFKR Sequence of the Cytoplasmic Domain of the Integrin IIb Subunit in a Patient With a Glanzmann’s Thrombasthenia-Like Syndrome

The integrin IIbβ3 mediates platelet aggregation through its fibrinogen and adhesive protein-binding properties. Particular interest concerns the role of the cytoplasmic domains of IIb and β3. We now report the molecular analysis of IIbβ3 from a patient with a Glanzmann’s thrombasthenia-like syndrome for whom the principal characteristics are an approximate 50% total platelet content of IIbβ3 but with a much lower proportion in the surface pool (Hardisty et al, Blood 80:696, 1992). Polymerase chain reaction (PCR) single-strand conformational polymorphism and DNA sequencing showed a heterozygous mutation giving rise to amino acid substitution R995 to Q in the GFFKR sequence of the cytoplasmic domain of IIb. Reverse transcriptase-PCR and polymorphism analysis only detected mRNA for the mutated allele of the IIb gene and a single allele of the β3 gene in his platelets, suggesting other unidentified defects. Site-directed mutagenesis followed by transient expression of the mutated IIb together with wild-type β3 in Cos-7 cells resulted in a markedly decreased expression of the complex at the cell surface when compared with cells transfected with wild-type IIb and β3. Flow cytometry with PAC-1 and a stable Chinese hamster ovary–transfected cell line showed that the mutated receptor was not locked into a high activation state, although it became so in the presence of the activating antibody, anti-LIBS6. This is the first reported natural mutation in the highly conserved GFFKR sequence of the IIb cytoplasmic domain.

A two-amino acid insertion in the Cys146- Cys167 loop of the alphaIIb subunit is associated with a variant of Glanzmann thrombasthenia. Critical role of Asp163 in ligand binding

The ligand binding site(s) of the alpha subunit of integrin alphaIIb beta3 (GPIIb-IIIa), a prototypic non-I domain integrin, remains elusive. In this study, we have characterized a Japanese variant of Glanzmann thrombasthenia, KO, whose platelets express normal amounts of alphaIIb beta3. KO platelets failed to bind the activation-independent ligand-mimetic mAb OP-G2 and did not bind fibrinogen or the activation-dependent ligand-mimetic mAb PAC-1 following activation of alphaIIb beta3 under any condition examined. Sequence analysis of PCR fragments derived from KO platelet mRNA revealed a 6-bp insertion leading to a 2-amino-acid insertion (Arg-Thr) between residues 160 and 161 of the alphaIIb subunit. Introduction of the insertion into wild-type recombinant alphaIIb beta3 expressed in 293 cells led to the normal expression of alphaIIb beta3 having the defect in ligand binding function. The insertion is located within the small loop (Cys146-Cys167) in the third NH2-terminal repeat of the alphaIIb subunit. Alanine substitution of each of the oxygenated residues within the loop (Thr150, Ser152, Glu157, Asp159, Ser161, and Asp163) did not significantly affect expression of alphaIIbbeta3, and only Asp163AlaalphaIIb beta3 abolished the ligand binding function. In addition, Asp163AlaalphaIIb beta3 as well as KO mutant alphaIIb beta3 constitutively expressed the PMI-1 epitope. Our present data suggest that Asp163 of the alphaIIb subunit is one of the critical residues for ligand binding.

Novel point mutations in the alphaIIb subunit (Phe289-->Ser, Glu324-->Lys and Gln747-->Pro) causing thrombasthenic phenotypes in four Japanese patients

We analysed the molecular basis of Glanzmann thrombasthenia (GT) in four Japanese patients with type I or type II disease. Polymerase chain reaction (PCR) and subsequent direct sequencing of platelet RNA and genomic DNA revealed three single nucleotide substitutions of the alphaIIb gene, which were confirmed by allele-specific PCR or restriction analysis. One patient with type I GT had a T to C base substitution in exon 11 resulting in a Phe (TTT)-289 to Ser (TCT) mutation (F289S) of the subunit. Another type I patient had a G to A base substitution in exon 12 resulting in a Glu (GAA)-324 to Lys (AAA) mutation (E324K). Interestingly, two unrelated patients with type II GT shared an A to C base substitution in exon 2 3, a region previously not associated with GT, resulting in a Gln (CAA)-747 to Pro (CCA) mutation (Q747P). To analyse the effects of these mutations on alphaII(b)beta3 surface expression, the wild-type alphaIIb cDNA or mutant alphaIIb cDNAs were transfected into Chinese hamster ovary (CHO) cells together with a wild-type beta3 cDNA. Flow cytometric analysis using an anti-alphaII(b)beta3 complex antibody revealed that 50.6% of CHO cells with wild-type alphaII(b)beta3 expressed complexes, whereas only 1 6%, 7.7% and 31.3% of cells, with IIb(F289S)beta3, alphaIIb(E324K)beta3 and alphaIIb(Q747P)beta3 expressed complexes, respectively. Our data indicate that these three novel point mutations in the alphaIIb subunit may hamper surface expression of the alphaII(b)beta3 complex, thus resulting in the quantitative GT phenotypes of platelets from these patients.

Regulation of integrin function: evidence that bivalent-cation-induced conformational changes lead to the unmasking of ligand-binding sites within integrin alpha5 beta1

The molecular mechanisms that regulate integrin-ligand binding are unknown; however, bivalent cations are essential for integrin activity. According to recent models of integrin tertiary structure, sites involved in ligand recognition are located on the upper face of the seven-bladed beta-propeller formed by the N-terminal repeats of the alpha subunit and on the von Willebrand factor A-domain-like region of the beta subunit. The epitopes of function-altering monoclonal antibodies (mAbs) cluster in these regions of the alpha and beta subunits; hence these mAbs can be used as probes to detect changes in the exposure or shape of the ligand-binding sites. Bivalent cations were found to alter the apparent affinity of binding of the inhibitory anti-alpha5 mAbs JBS5 and 16, the inhibitory anti-beta1 mAb 13, and the stimulatory anti-beta1 mAb 12G10 to alpha5 beta1. Analysis of the binding of these mAbs to alpha5beta1 over a range of Mn2+, Mg2+ or Ca2+ concentrations demonstrated that there was a concordance between the ability of cations to elicit conformational changes and the ligand-binding potential of alpha5 beta1. Competitive ELISA experiments provided evidence that the domains of the alpha5 and beta1 subunits recognized by mAbs JBS5/16 and 13/12G10 are spatially close, and that the distance between these two domains is increased when alpha5 beta1 is occupied by bivalent cations. Taken together, our findings suggest that bivalent cations induce a conformational relaxation in the integrin that results in exposure of ligand-binding sites, and that these sites lie near an interface between the alpha subunit beta-propeller and the beta subunit putative A-domain.

Truncation of the cytoplasmic domain of beta3 in a variant form of Glanzmann thrombasthenia abrogates signaling through the integrin alpha(IIb)beta3 complex

Glanzmann thrombasthenia is an inherited bleeding disorder characterized by absence or dysfunction of the platelet integrin alpha(IIb)beta3. Patient RM is a thrombasthenic variant whose platelets fail to aggregate in response to physiological agonists, despite the fact that they express abundant levels of alpha(IIb)beta3 on their surface. Binding of soluble fibrinogen or fibrinogen mimetic antibodies to RM platelets did not occur, except in the presence of ligand-induced binding site (LIBS) antibodies that transformed the RM integrin complex into an active conformation from outside the cell. Sequence analysis of PCR-amplified genomic DNA and platelet mRNA revealed a C2268T nucleotide substitution in the gene encoding the integrin beta3 subunit that resulted in an Arg724Ter mutation, producing a truncated protein containing only the first eight of the 47 amino acids normally present in the cytoplasmic domain. Functional analysis of both RM platelets and CHO cells stably expressing this truncated integrin revealed that the alpha(IIb)beta3Arg724Ter complex is able to mediate binding to immobilized fibrinogen, though downstream events, including cytoskeletally-mediated cell spreading and tyrosine phosphorylation of focal adhesion kinase, pp125FAK, fail to occur. These studies establish the importance of the membrane-distal portion of the integrin beta3 cytoplasmic domain in bidirectional transmembrane signaling in human platelets, and the role of integrin signaling in maintaining normal hemostasis in vivo.

A Leu117 → Trp Mutation Within the RGD-Peptide Cross-Linking Region of β3 Results in Glanzmann Thrombasthenia by Preventing αIIbβ3 Export to the Platelet Surface

We report a case of Glanzmann thrombasthenia in a Pakistani child whose platelets express less than 10% of the normal amount of αIIbβ3 on their surface. Single-stranded conformation polymorphism analysis of the exons of the patient's αIIb and β3 genes showed an abnormality in exon 4 of the β3 gene. Direct sequence analysis showed that the patient was homozygous for a T → G nucleotide substitution in this exon, resulting in the replacement of a highly conserved Leu at position 117 with Trp. Heterologous expression of αIIbβ3 containing the β3 mutation in COS-1 cells confirmed the pathogenicity of the Leu117 → Trp substitution and showed that it resulted in the intracellular retention of malfolded αIIbβ3 heterodimers. Additional site-directed mutagenesis at position 117 indicated that, although the smaller hydrophobic amino acid Val could be substituted for the wild-type Leu, the larger hydrophobic amino acids Trp and Phe or the charged amino acids Asp and Lys were not tolerated. These studies indicate that Leu117 in β3 plays a critical role in attaining the correct folded conformation of αIIbβ3. These studies also suggest that the hydrophobic side chain of Leu117 is likely folded into the interior of β3, where it serves to stabilize internal packing of the protein and determines its overall shape.

A Three Amino Acid Deletion in Glycoprotein IIIa Is Responsible for Type I Glanzmann's Thrombasthenia: Importance of Residues Ile325Pro326Gly327 for β3 Integrin Subunit Association

Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by abnormal platelet aggregation due to a qualitative or quantitative defect of the glycoprotein (GP) IIb-IIIa complex (integrin αIIbβ3). We describe a new mutation in the GPIIIa gene responsible for type I GT in a consanguineous Algerian family. A discordance between phenotyping and genotyping of the GPIIIa-related HPA-1 platelet alloantigen system in three family members heterozygous for the disease suggested a genetic defect in the GPIIIa gene and a normal GPIIb gene. Sequence analysis of amplified genomic DNA fragments showed a 6-bp deletion in exon 7 of the GPIIIa gene resulting in the amino acid deletion/substitution (Ile325Pro326Gly327 → Met) and creating a new BspHI restriction site. Expression of the mutated integrin β3 subunit cDNA in Chinese hamster ovary cells showed that the cDNA gene was transcribed into a full-length β3 protein with an apparent molecular weight identical to wild-type β3 and accumulated as a single-chain molecule in the cell cytoplasm. The absence of heterodimeric complex formation of the mutant β3 protein with endogeneous αv was shown by immunoprecipitation experiments, intracellular immunofluorescent labeling, and a semiquantitative enzyme-linked immunosorbent assay using the αvβ3 complex-specific monoclonal antibodies LM609 and 23C6. Substitution of the methionine residue by a proline, present at position 326 of wild-type β3, did not restore the ability of the recombinant mutant β3 protein to associate with αv, suggesting that the Ile-Pro-Gly motif is located in a β3 domain important for integrin subunit interaction. The association of a BspHI restriction site with this newly identified mutation has allowed allele-specific restriction analysis of Algerian GT individuals and the identification of two new unrelated type I patients exhibiting the same mutation, suggesting that the described mutation might be significant in this population and that BspHI restriction analysis will provide a useful screening assay for antenatal diagnosis and genetic counselling.

A Three Amino Acid Deletion in Glycoprotein IIIa Is Responsible for Type I Glanzmann's Thrombasthenia: Importance of Residues Ile325Pro326Gly327 for β3 Integrin Subunit Association

Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by abnormal platelet aggregation due to a qualitative or quantitative defect of the glycoprotein (GP) IIb-IIIa complex (integrin αIIbβ3). We describe a new mutation in the GPIIIa gene responsible for type I GT in a consanguineous Algerian family. A discordance between phenotyping and genotyping of the GPIIIa-related HPA-1 platelet alloantigen system in three family members heterozygous for the disease suggested a genetic defect in the GPIIIa gene and a normal GPIIb gene. Sequence analysis of amplified genomic DNA fragments showed a 6-bp deletion in exon 7 of the GPIIIa gene resulting in the amino acid deletion/substitution (Ile325Pro326Gly327 → Met) and creating a new BspHI restriction site. Expression of the mutated integrin β3 subunit cDNA in Chinese hamster ovary cells showed that the cDNA gene was transcribed into a full-length β3 protein with an apparent molecular weight identical to wild-type β3 and accumulated as a single-chain molecule in the cell cytoplasm. The absence of heterodimeric complex formation of the mutant β3 protein with endogeneous αv was shown by immunoprecipitation experiments, intracellular immunofluorescent labeling, and a semiquantitative enzyme-linked immunosorbent assay using the αvβ3 complex-specific monoclonal antibodies LM609 and 23C6. Substitution of the methionine residue by a proline, present at position 326 of wild-type β3, did not restore the ability of the recombinant mutant β3 protein to associate with αv, suggesting that the Ile-Pro-Gly motif is located in a β3 domain important for integrin subunit interaction. The association of a BspHI restriction site with this newly identified mutation has allowed allele-specific restriction analysis of Algerian GT individuals and the identification of two new unrelated type I patients exhibiting the same mutation, suggesting that the described mutation might be significant in this population and that BspHI restriction analysis will provide a useful screening assay for antenatal diagnosis and genetic counselling.

Emerging paradigms of integrin ligand binding and activation

Adhesion of cells to each other or to the extracellular matrix provides essential signals that regulate many cellular functions including cell migration, proliferation, differentiation and apoptosis. The integrin superfamily orchestrates many of these complex adhesive events through regulated interactions with a large variety of ligands. Crystallization of some ligands and of a ligand-binding integrin domain, reviewed here, together with extensive mutagenesis studies are beginning to shed light on the inner workings of these receptors.