Characterization of the human platelet glycoprotein IIIa gene. Comparison with the fibronectin receptor beta-subunit gene

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.

Platelets and cancer: a casual or causal relationship: revisited

Human platelets arise as subcellular fragments of megakaryocytes in bone marrow. The physiologic demand, presence of disease such as cancer, or drug effects can regulate the production circulating platelets. Platelet biology is essential to hemostasis, vascular integrity, angiogenesis, inflammation, innate immunity, wound healing, and cancer biology. The most critical biological platelet response is serving as "First Responders" during the wounding process. The exposure of extracellular matrix proteins and intracellular components occurs after wounding. Numerous platelet receptors recognize matrix proteins that trigger platelet activation, adhesion, aggregation, and stabilization. Once activated, platelets change shape and degranulate to release growth factors and bioactive lipids into the blood stream. This cyclic process recruits and aggregates platelets along with thrombogenesis. This process facilitates wound closure or can recognize circulating pathologic bodies. Cancer cell entry into the blood stream triggers platelet-mediated recognition and is amplified by cell surface receptors, cellular products, extracellular factors, and immune cells. In some cases, these interactions suppress immune recognition and elimination of cancer cells or promote arrest at the endothelium, or entrapment in the microvasculature, and survival. This supports survival and spread of cancer cells and the establishment of secondary lesions to serve as important targets for prevention and therapy.

The glycoprotein IIIa gene polymorphism and the risk of myocardial infarction

The hemostatic platelet response of an individual may be influenced by the genetic profile of the platelet membrane glycoprotein (GP) receptors. The GP IIIa, as a part of von Willebrand factor and fibrinogen receptor, plays a central role in platelet aggregation. Polymorphism in GP IIIa has been extensively studied for its association with myocardial infarction or coronary artery diseases. To date, the role of GP IIIa polymorphism in genetic susceptibility to thrombotic disease still remains controversial. The results of case-control association studies vary, even with the same ethnic background, and the association was hardly found in studies with larger sample size, suggesting publication-bias toward positive findings. In meta-analysis, the GP IIIa Pl(A2) allele carriers did not show increased risk for myocardial infarction compared with Pl(A1/A1) homozygotes. The functional studies also showed conflicting results. In conclusion, GP IIIa Pl(A1/A2) polymorphism does not seem to have a major role either in determining the individual variance of platelet function or the risk of myocardial infarction according to the currently available data. Therefore, the genotype determination of GP IIIa Pl(A1/A2 )polymorphism may not be useful for risk assessment of myocardial infarction at this time. Nevertheless, the author can not completely exclude its possible role in coronary thrombosis after angioplasty or sudden cardiac death. Thus, further evaluation in larger prospective or multicenter studies is required to elucidate the role of GPs in cardiovascular system.

Molecular characterization and expression analysis of the gene coding for the porcine beta(3) integrin subunit (CD61)

Integrins are heterodimeric cell adhesion molecules with major roles in a variety of biological processes ranging from cell migration to tissue organization, immune and non-immune defense mechanisms and oncogenic transformation. Members of the beta(3) integrin subfamily are composed of a beta(3) subunit (CD61) non-covalently associated with two alpha subunits, alpha(IIb) (CD41) and alpha(v) (CD51), to constitute a group of transmembrane glycoproteins that participate in many physiologically important events. This investigation has focused on the molecular characterization of the cDNA encoding the porcine beta(3) integrin subunit. The deduced 762-amino acid sequence was 93, 92, 91, 89, 79 and 73% homologous to human, dog, rabbit, mouse, chicken and Xenopus laevis CD61 protein, respectively. Porcine CD61 molecule shares many structural features with human CD61, including a region containing a metal ion-dependent adhesion site (MIDAS) folding into an I domain-like structure. Through PCR-SSCP analysis and sequencing, six polymorphic positions were detected in the cDNA sequence of porcine CD61, and their frequencies were observed from a collection of 47 pigs. Expression analysis was done at two different levels: expression of the CD61 mRNA by RT-PCR and localization of the protein by immunohistochemistry. Our results show that CD61 transcripts were detected mainly in platelets and hematopoietic tissues. The immunohistochemical tissue localization of CD61 protein by a specific monoclonal antibody against CD61 recombinant protein showed that CD61 was expressed on vascular and non-vascular smooth muscle, epithelium and myeloid cells, being undetectable in cells of the lymphoid lineage. Furthermore, pulmonary intravascular macrophages (PIM), a subpopulation of macrophages which seem to play an important role in blood clearance, expressed much more CD61 when compared to pulmonary alveolar macrophages (PAM). The knowledge of the structure and distribution of the CD61 provides insight into the physiological function of the porcine beta(3) integrins and should be of importance in understanding the role of this integrin family in biological processes.

Characterization of the cDNA Encoding alphaIIb and beta3 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.

Nucleotide sequence of the canine alphaIIb gene from platelet-derived cDNA

OBJECTIVE

To determine the nucleotide sequence of the alphaIIb gene from canine platelet-derived cDNA.

ANIMALS

3 adult dogs.

PROCEDURE

First-strand cDNA was prepared from total RNA isolated from canine platelets. The cDNA was amplified, using specific primers in polymerase chain reaction (PCR), and the nucleotide sequence was obtained from purified PCR products.

RESULTS

Except for the nucleotide at position 694, results of all sequencing reactions of alphaIIb were identical for canine platelet-derived cDNA. Canine alphaIIb had 3 fewer codons than alphaIIb of humans. The nucleotide and deduced amino acid sequences of full-length canine alphaIIb shared > or = 83% similarity with the sequences established for humans. Segments of canine alphaIIb nucleotide and deduced amino acid sequences were > or = 78% similar to alphaIIb associated with 7 functional domains (extracellular, transmembrane, cytoplasmic, and 4 calcium-binding domains) in humans, with the highest degree of similarity correlating with the sequences of the 4 calcium-binding domains. Amino acid residues associated with development of alloantibodies in humans (Met837, Val837, Ile843, Ser843) are not encoded by canine alphaIIb.

CONCLUSIONS AND CLINICAL RELEVANCE

The nucleotide variation at position 694 of canine alphaIIb may represent a polymorphism. The species differences in the alphaIIb sequence may contribute to variations in receptor-li gand interactions. The high degree of alphaIIb sequence conservation of the 4 calcium-binding domains implies functional importance. Some disorders associated with alphaIIbbeta3 in dogs are clinically analogous to diseases in humans, and results indicate that dogs are an appropriate model for the evaluation of gene therapy and other treatments of platelet-associated disorders.

Competition between normal [674C] and mutant [674R] subunits: role of the molecular chaperone BiP in the processing of GPIIb-IIIa complexes

This work aimed at investigating the function of the [C674R] mutation in GPIIb that disrupts the intramolecular 674 to 687 disulfide bridge. Individuals heterozygous for this mutation show a platelet GPIIb-IIIa content approximately 30% of normal controls, which is less than expected from one normal functioning allele. Coexpression of normal [674C]GPIIb and mutant [674R]GPIIb with normal GPIIIa produced a [674R]GPIIb concentration-dependent inhibition of surface exposure of GPIIb-IIIa complexes in Chinese hamster ovary (CHO) cells, suggesting that [674R]GPIIb interferes with the association and/or intracellular trafficking of normal subunits. Mutation of either 674C or 687C had similar effects in reducing the surface exposure of GPIIb-IIIa. However, substitution of 674C for A produced a much lesser inhibition than R, suggesting that a positive-charged residue at that position renders a less efficient subunit conformation. The mutant [674R]GPIIb but not normal GPIIb was found associated with the endoplasmic reticulum chaperone BiP in transiently transfected CHO cells. BiP was also found associated with [674R]GPIIb-IIIa heterodimers, but not with normal GPIIIa or normal heterodimers. Overexpression of BiP did not increase the surface exposure of [674R]GPIIb-IIIa complexes, indicating that its availability was not a limiting step. Platelets from the thrombasthenic patient expressing [674R]GPIIb-IIIa were found to bind soluble fibrinogen in response to physiologic agonists or dithiothreitol treatment. Thus, the [674R]GPIIb mutation leads to a retardation of the secretory pathway, most likely related to its binding to the molecular chaperone BiP, with the result of a defective number of functional GPIIb-IIIa receptors in the cell surface.

Two genetic defects in alphaIIb are associated with type I Glanzmann's thrombasthenia in a Great Pyrenees dog: a 14-base insertion in exon 13 and a splicing defect of intron 13

Glannzmann's thrombasthenia (GT) is an autosomal recessive bleeding disorder caused by qualitative or quantitative deficiencies of the platelet membrane glycoprotein alphaIIbbeta3. This is the first report of a molecular genetic basis for type I GT in dogs. As previously reported, a thrombasthenic Great Pyrenees dog (dog No. 1) experienced uncontrolled epistaxis despite results of coagulation screening tests, platelet quantitation, and von Willebrand factor quantitation that were within reference ranges. Platelet aggregation was minimal in response to agonists. Flow cytometry, autoradiography, and immunoblot experiments demonstrated either marked reduction or absence of glycoproteins alphaIIb and beta3. In this study, we report the presence of a 14-base insertion in exon 13 and defective splicing of intron 13 in the alphaIIb gene of two thrombasthenic dogs (Nos. 1 and 8). The insertion disrupted the fourth alphaIIb calcium-binding domain, caused a shift in the reading frame and resulted in a premature termination codon. Possible consequences of this mutation include decreased alphaIIb mRNA stability and production of truncated alphaIIb protein that lacks the transmembrane and cytoplasmic domains and a large portion of the extracellular domain. We identified the dam, sire, and three littermates of dog No. 8 as carriers of the alphaIIb mutation. Canine alphaIIb and beta3 genes share significant homology with the genes in human beings, making canine GT an excellent translational model for human GT. A defined molecular basis for canine GT will enhance ongoing gene therapy research and increase the understanding of structure-function relationships of this integrin.

Development and validation of a genotyping kit for the eight principal human platelet alloantigen systems

Thrombocytopenia in newborns is often due to maternal alloimmunization against platelet alloantigens of the foetus which have been inherited from the father and are absent in the mother. Our aim was to develop a "ready-to-use" typing kit based on polymerase chain reactions, using sequence-specific primers for rapid and simultaneous genotyping of the eight principal human platelet alloantigens. The typing technique uses two specific primer pairs for each bi-allelic system and a monomorphic primer pair as amplification control, with a single temperature cycle programme and identical PCR stringency conditions for all pairs of primers. This kit allows typing of blood samples of small volume within three hours after reception. Validation criteria are essential to check the reliability and specificity of the test, and DNA controls carrying the targeted HPA alleles must be obtained from typed individuals or created in vitro by PCR.

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.

Integrins: alternative splicing as a mechanism to regulate ligand binding and integrin signaling events

Integrins are a family of transmembrane proteins composed of heterodimers of alpha and beta subunits. With their extracellular domain they bind extracellular matrix proteins or other cell surface molecules, and their cytoplasmic domain binds to cytoskeletal and signaling proteins. Thus, they are in an ideal position to transfer information from the extracellular environment to the interior of the cell and vice versa. For several integrin subunits, alternative splicing of mRNA leads to variations in the sequence of both extracellular and cytoplasmic domains. Many integrin splice variants have specific expression patterns, but for some time, functional differences between these variants were not evident. Recent experiments using transfected cell lines and gene targeting of specific splice variants have contributed significantly to our understanding of the function of these splice variants. The results indicate that alternative splicing is a mechanism to subtly regulate the ligand binding and signaling activity of integrins.

Molecular Genetic Analysis of a Compound Heterozygote for the Glycoprotein (GP) IIb Gene Associated With Glanzmann’s Thrombasthenia: Disruption of the 674-687 Disulfide Bridge in GPIIb Prevents Surface Exposure of GPIIb-IIIa Complexes

This work was aimed at elucidating the molecular genetic lesion(s) responsible for the thrombasthenic phenotype of a patient whose low platelet content of glycoprotein (GP) IIb-IIIa indicated that it was a case of type II Glanzmann’s thrombasthenia (GT). The parents did not admit consanguinity and showed a reduced platelet content of GPIIb-IIIa. Polymerase chain reaction (PCR)–single-stranded conformational polymorphism analysis of genomic DNA showed no mutations in the patient’s GPIIIa and two novel mutations in the GPIIb gene: one of them was a heterozygous splice junction mutation, a C→A transversion, at position +2 of the exon 5-intron 5 boundary [IVS5(+2)C→A] inherited from the father. The predicted effect of this mutation, insertion of intron 5 (76 bp) into the GPIIb-mRNA, was confirmed by reverse transcription-PCR analysis of platelet mRNA. The almost complete absence of this mutated form of GPIIb-mRNA suggests that it is very unstable. Virtually all of the proband’s GPIIb-mRNA was accounted for by the allele inherited from the mother showing a T2113→C transition that changes Cys674→Arg674 disrupting the 674-687 intramolecular disulfide bridge. The proband showed a platelet accumulation of proGPIIb and minute amounts of GPIIb and GPIIIa. Moreover, transfection and immunoprecipitation analysis demonstrated that [Arg674]GPIIb is capable of forming a heterodimer complex with GPIIIa, but the rate of subunit maturation and the surface exposure of GPIIb-IIIa are strongly reduced. Thus, the intramolecular 674-687 disulfide bridge in GPIIb is essential for the normal processing of GPIIb-IIIa complexes. The additive effect of these two GPIIb mutations provides the molecular basis for the thrombasthenic phenotype of the proband.

Molecular Genetic Analysis of a Compound Heterozygote for the Glycoprotein (GP) IIb Gene Associated With Glanzmann’s Thrombasthenia: Disruption of the 674-687 Disulfide Bridge in GPIIb Prevents Surface Exposure of GPIIb-IIIa Complexes

This work was aimed at elucidating the molecular genetic lesion(s) responsible for the thrombasthenic phenotype of a patient whose low platelet content of glycoprotein (GP) IIb-IIIa indicated that it was a case of type II Glanzmann’s thrombasthenia (GT). The parents did not admit consanguinity and showed a reduced platelet content of GPIIb-IIIa. Polymerase chain reaction (PCR)–single-stranded conformational polymorphism analysis of genomic DNA showed no mutations in the patient’s GPIIIa and two novel mutations in the GPIIb gene: one of them was a heterozygous splice junction mutation, a C→A transversion, at position +2 of the exon 5-intron 5 boundary [IVS5(+2)C→A] inherited from the father. The predicted effect of this mutation, insertion of intron 5 (76 bp) into the GPIIb-mRNA, was confirmed by reverse transcription-PCR analysis of platelet mRNA. The almost complete absence of this mutated form of GPIIb-mRNA suggests that it is very unstable. Virtually all of the proband’s GPIIb-mRNA was accounted for by the allele inherited from the mother showing a T2113→C transition that changes Cys674→Arg674 disrupting the 674-687 intramolecular disulfide bridge. The proband showed a platelet accumulation of proGPIIb and minute amounts of GPIIb and GPIIIa. Moreover, transfection and immunoprecipitation analysis demonstrated that [Arg674]GPIIb is capable of forming a heterodimer complex with GPIIIa, but the rate of subunit maturation and the surface exposure of GPIIb-IIIa are strongly reduced. Thus, the intramolecular 674-687 disulfide bridge in GPIIb is essential for the normal processing of GPIIb-IIIa complexes. The additive effect of these two GPIIb mutations provides the molecular basis for the thrombasthenic phenotype of the proband.

Truncation of Glycoprotein (GP) IIIa (▵ 616-762) Prevents Complex Formation With GPIIb: Novel Mutation in Exon 11 of GPIIIa Associated With Thrombasthenia

This work reports the molecular genetic study of a patient who suffered from Glanzmann thrombasthenia (GT). Structural analysis of the glycoprotein (GP) IIb and GPIIIa genes showed the presence of a homozygous G1846→T transversion in exon 11 of GPIIIa that changes Glu616→Stop. Cytometric and immunochemical analysis indicated that platelet GPIIb-IIIa was absent in the proband but present at normal levels in the heterozygous relatives. The following observations indicate that this mutation is responsible for the thrombasthenic phenotype of the proband. (1) We failed to detect mutations other than [T1846]GPIIIa in the coding region of both GPIIb and GPIIIa genes. (2) The G1846→T mutation was observed in either parent and a brother of the proband, but none of 100 unrelated individuals carried this defect. (3) Pulse-chase and immunoprecipitation analysis of GPIIb-IIIa complexes in cells transiently cotransfected with cDNAs encoding normal GPIIb and [T1846]GPIIIa showed neither maturation of GPIIb nor complex formation and surface exposure of GPIIb-▵GPIIIa. These observations indicate that the sequence from Glu616 to Thr762 in GPIIIa is essential for heterodimerization with GPIIb. Polymerase chain reaction-based analysis demonstrated the presence of normal levels of full-length GPIIIa-mRNA in the proband and in heterozygous relatives. In addition, a shortened transcript, with a 324-nucleotide deletion, resulting from in-frame skipping of exons 10 and 11, was detectable upon reamplification of the DNA. Thus, unlike other nonsense mutations, [T1846]GPIIIa does not lead to abnormal processing or reduction in the number of transcripts with the termination codon.

Truncation of Glycoprotein (GP) IIIa (▵ 616-762) Prevents Complex Formation With GPIIb: Novel Mutation in Exon 11 of GPIIIa Associated With Thrombasthenia

This work reports the molecular genetic study of a patient who suffered from Glanzmann thrombasthenia (GT). Structural analysis of the glycoprotein (GP) IIb and GPIIIa genes showed the presence of a homozygous G1846→T transversion in exon 11 of GPIIIa that changes Glu616→Stop. Cytometric and immunochemical analysis indicated that platelet GPIIb-IIIa was absent in the proband but present at normal levels in the heterozygous relatives. The following observations indicate that this mutation is responsible for the thrombasthenic phenotype of the proband. (1) We failed to detect mutations other than [T1846]GPIIIa in the coding region of both GPIIb and GPIIIa genes. (2) The G1846→T mutation was observed in either parent and a brother of the proband, but none of 100 unrelated individuals carried this defect. (3) Pulse-chase and immunoprecipitation analysis of GPIIb-IIIa complexes in cells transiently cotransfected with cDNAs encoding normal GPIIb and [T1846]GPIIIa showed neither maturation of GPIIb nor complex formation and surface exposure of GPIIb-▵GPIIIa. These observations indicate that the sequence from Glu616 to Thr762 in GPIIIa is essential for heterodimerization with GPIIb. Polymerase chain reaction-based analysis demonstrated the presence of normal levels of full-length GPIIIa-mRNA in the proband and in heterozygous relatives. In addition, a shortened transcript, with a 324-nucleotide deletion, resulting from in-frame skipping of exons 10 and 11, was detectable upon reamplification of the DNA. Thus, unlike other nonsense mutations, [T1846]GPIIIa does not lead to abnormal processing or reduction in the number of transcripts with the termination codon.

Inherited platelet glycoprotein polymorphisms and a risk for coronary heart disease in young central Europeans

Among central Europeans polymorphisms of GPIIIa, GPIb, GPIIb, and GPIa named human platelet antigen-1 (HPA-1), -2, -3, and -5 are the clinically most relevant systems in which alloimmunization occurs. These genetically determined polymorphisms of glycoproteins may render platelets sensible for plaque formation and thus could increase risk for coronary artery disease (CAD). We therefore determined gene frequencies of HPA-1, -2, -3, and -5 in European patients suffering from CAD (n = 92; median age, 46 years) or CAD accompanied by diabetes mellitus (DM) (n = 30; median age, 60 years, DM I/II, 5/25) and compared the data obtained with those in DM patients without CAD (n = 80; median age, 43 years; DM I/II, 53/27) and a control group (newborns, n = 906). Triglyceride and cholesterin levels as well as the percentage of smokers was significantly higher in the CAD group compared with the diabetics without DM (p < 0.005). No significant difference of the frequencies of any HPA-type between CAD patients with or without DM, diabetics, or controls could be detected. This was also true when evaluating a subgroup of patients aged 45 years or younger. To include a mutual influence of the described HPA-polymorphisms, we condensed the four HPA genotypes to joint glycoprotein variants. Again the same frequencies were found in patient groups and controls, when analyzing the five most common condensed joint glycoprotein variants. The analysis of the combined published studies shows that the pooled HPA-1 allele frequencies are identical in controls and CAD patients. Thus, no significant association between the polymorphisms of any of the studied HPA systems and the development of CAD can be found in central Europeans.

Identification of a region in the integrin beta3 subunit that confers ligand binding specificity

Many integrin adhesion receptors bind ligands containing the Arg-Gly-Asp (RGD) peptide motif. Most integrins exhibit considerable specificity for particular ligands and can distinguish among the many conformations of RGD. In this study we identify the domain of the integrin beta subunit involved in determining ligand binding specificity. Chimeras of beta3 and beta5, the most homologous integrin beta subunits, were expressed with alphav on the surface of human 293 cells. The ligand binding phenotype of each chimera was assessed using the ligands Fab-9 and fibrinogen, both of which have a binding preference for alphavbeta3. The results of the study show that when exons C and D of the beta3 subunit (residues 95-233) are substituted into beta5, the chimera gained the ability to bind Fab-9 with an affinity close to that of wild-type alphavbeta3. This chimera was able to mediate cell adhesion to fibrinogen. Furthermore, the swap of only a 39-residue segment of this larger domain, beta3 residues 164-202, into the backbone of beta5 enabled the chimeric integrin to bind soluble Fab-9. This small domain is highly divergent among the integrin beta subunits, suggesting that it may play a role in determining ligand selection by all integrins.

Integrin alphaIIb beta3 reconstituted into lipid bilayers is nonclustered in its activated state but clusters after fibrinogen binding

Integrin activation, ligand binding, and integrin clustering were analyzed using alphaIIb beta3 reconstituted into phospholipid vesicles and into supported planar lipid bilayers. Strong and specific binding of fibrinogen and the gamma-chain dodecapeptide of fibrinogen to alphaIIb beta3 indicated that the integrin is in an activated state after membrane reconstitution. Cryoelectron and fluorescence microscopy suggested a nonclustered state of the protein in the vesicle membrane. Supported planar lipid membranes were generated by fusion of vesicles in which approximately equal fractions of integrins were pointing inside-out and outside-in. This distribution led to an immobilization of about 40% of the integrin in supported bilayers due to attachment of the large extracellular domains to the quartz support. Fluorescence recovery after photobleaching indicated a diffusion coefficient of D = (0.70 +/- 0.06) x 10(-8) cm2/s, consistent with a nonclustered state of the mobile integrin. Upon fibrinogen binding, the integrins became immobile, and fluorescence micrographs showed a patchy distribution of fibrinogen-integrin complexes consisting of approximately 250 molecules. In addition to the expected dimer formation by bivalent fibrinogen, additionally induced fibrinogen clustering may account for the large size of the complexes. In contrast, binding of monovalent GRGDS pentapeptide or the gamma-chain dodecapeptide of fibrinogen altered neither the mobile fraction nor the association state of alphaIIb beta3. Our data indicate that integrin alphaIIbb3 is activated while monodisperse, and became clustered upon fibrinogen binding, leading to an irreversibly bound state.

HPA-10wb (Laa): Genetic Determination of a New Platelet-Specific Alloantigen on Glycoprotein IIIa and Its Expression in COS-7 Cells

The heterodimeric complex glycoprotein (GP)IIb-IIIa, the fibrinogen receptor of platelets, carries numerous alloantigen systems. These polymorphisms are responsible for the immune response after transfusion or during pregnancy. In the latter case, the mother develops an antibody against an epitope present on fetal platelets, and this results in platelet destruction in the fetus. In this report, we describe the molecular characterization of a new alloantigen (Laa) on GPIIIa responsible for neonatal alloimmune thrombocytopenia (NAIT). Using polymerase chain reaction (PCR)–singlestrand conformation polymorphism (SSCP) and DNA sequencing, we found a point mutation (G to A) in a heterozygous state on the GPIIIa gene leading to amino acid substitution Arg to Gln at position 62 of the mature protein. Transient expression of GPIIb-IIIa complexes in Cos-7 cells using wild-type or mutated GPIIIa cDNA allowed us to demonstrate that this mutation was responsible for expression of the Laa epitope.

Critical residues for ligand binding in an I domain-like structure of the integrin beta1 subunit

Several integrin alpha subunits have an inserted sequence of about 200 residues (the I or A domain) that is critical for ligand interactions. The presence of an I domain-like structure within the integrin beta subunit has been proposed based on the similarity of the hydropathy profiles and the homology of sequences between the alpha and beta subunits. This study was designed to determine whether the region of the beta1 subunit that includes residues 101-335 has the characteristics of an I domain. We found novel critical residues for ligand binding (Ser-132, Asn-224, Asp-226, Glu-229, Asp-233, Asp-267, and Asp-295, in addition to the previously reported Asp-130) using site-directed mutagenesis. The critical residues for ligand binding are located in several of loop structures of the region (or in a potential loop between an alpha helix and a beta strand), which have been predicted using multiple secondary structure prediction methods. The data suggest that the beta subunit has multiple disrupted critical oxygenated residues for ligand binding similar to those found in the alpha I domain.

Rapid typing for human platelet antigen systems-1, -2, -3 and -5 by PCR amplification with sequence-specific primers

Typing for human platelet antigens (HPA) is useful in a variety of clinical situations. We developed a method for genotyping for HPA-1, -2, -3 and -5 by means of the PCR amplification with sequence-specific primers (PCR-SSP) technique. Primer sets were designed to allow PCR amplification for all systems using the same assay conditions. Specificity and sensitivity of the method were assessed in a blind quality control study (n = 112). In 111 cases, results obtained by PCR-SSP were identical as compared with PCR-restriction fragment length polymorphism technique. One discrepancy was found to be due to a typing error in the data sheet. The results of the PCR-SSP technique were available within 3 h. We conclude that genotyping based on PCR-SSP enables rapid typing for HPA systems, which makes this technique feasible in most clinical settings where urgent HPA typing is required.

Genomic organization of the mouse beta 1 gene: conservation of the beta 1D but not of the beta 1B and beta 1C integrin splice variants

We have determined the genomic organization of the 3'-region of the murine beta 1 gene and cloned the murine beta 1D integrin splice variant. Overlapping genomic clones encompassing the region of the beta 1D-specific exons were isolated from a phage lambda FIXII library, mapped and partially sequenced. All of the exon-intron junctions identified in the murine beta 1 gene fit with the consensus splice donor and acceptor sequences and occur at the same positions as in their human counterparts. cDNA clones for the beta 1D integrin were isolated from a murine skeletal muscle library. The human and murine beta 1D sequences are conserved at the nucleotide (93%) and amino acid (100%) level, suggesting an important role of this muscle-specific variant throughout mammalian phylogenesis. In contrast, murine sequences for beta 1B are very different from human beta 1B at both the nucleotide as well as amino acid level. Moreover, no specific polyadenylation signal for the beta 1B variant could be identified in genomic clones, suggesting that this variant is not present in the mouse. Finally, we were not able to identify a murine beta 1C splice variant by sequencing analysis, Southern hybridization techniques or polymerase chain reaction of mRNA from platelets. These findings indicate that the beta 1B and beta 1C variants emerged relatively late in the phylogenesis of the beta 1 integrin family.

The genomic structure of the murine alpha 4 integrin gene

The VLA-4 (alpha 4 beta 1) integrin is a leukocyte glycoprotein involved in both cell-extracellular matrix and cell-cell interactions. We report here the cloning of the murine alpha 4 gene whose protein product is antigenically related to the human VLA-4 alpha chain. The alpha 4 m gene is about 75 kb long and consists of 28 exons, ranging in size from 46 bp (exon 13) to 437 bp (exon 1). The introns varied from 79 bp (intron 8) to more than 17 kb (intron 2). Three mRNA transcripts from this alpha 4 m gene can be visualized on Northern blot. After cloning the 3' untranslated region (3' UTR), four polyadenylation sites could be identified, presumably responsible for the presence of three to four transcripts of the alpha 4 gene, differing substantially in length.

A novel beta 1 integrin isoform produced by alternative splicing: unique expression in cardiac and skeletal muscle

The mRNA's of several integrin subunits are alternatively spliced in the region encoding cytoplasmic domains, that may potentially provide alternative integrin-cytoskeleton interactions and transmembrane signaling pathways. We identified a novel cytoplasmic tail variant of the human beta 1 subunit by reverse transcriptase polymerase chain reaction. This fourth beta 1 variant, named beta 1D, is specific for skeletal and cardiac muscle. The determined genomic organization of the 3'-region of the human beta 1 gene reveals that beta 1D is produced by alternative splicing of mRNA. In addition, we show that the expression of beta 1D is developmentally regulated during murine myoblast differentiation, suggesting a role for beta 1D in myogenesis.

Non-radioactive SSCP for genotyping human platelet alloantigens

Human platelet alloantigen systems are responsible for neonatal and post-transfusional thrombocytopenias. The determination of the different allotypes can be performed using immunological or DNA-based methods. The most used DNA-based procedure requires the digestion by specific restriction enzymes of PCR products containing the genetic determinants of these alloantigens. We now report a rapid method of genotyping which does not use restriction enzymes and is less prone to misinterpretation. This is non-radioactive PCR-SSCP (single strand conformation polymorphism), which we illustrate for two different HPA systems, one on GPIIIa (HPA-1) and the other on GPIIb (HPA-3).

Platelet glycoprotein IIb gene expression as a model of megakaryocyte-specific expression

Glycoprotein (GP)IIb/IIIa is an integrin complex normally restricted in its expression to platelets and the megakaryocytes from which they are derived. This complex functions as a receptor for fibrinogen and other ligands and is involved in platelet aggregation. The receptor complex is expressed at high levels during final megakaryocyte differentiation. Further, while GPIIIa is expressed in other tissues as part of the vitronectin receptor, GPIIb is only expressed on maturing megakaryocytes and the platelets derived from them. Thus studies of the GPIIb gene may serve as a model of gene regulation during this process. Over the past several years, the genes for both GPIIb and IIIa have been cloned and analyzed. The GPIIb gene contains 30 exons over 18 kilobases (kb). The transcriptional start site has been determined and there does not appear to be a TATA-box immediately upstream of this site. Studies have been done to define regulatory elements upstream of the transcriptional start site. Most of these studies focused on the human promoter and on studies using megakaryocytic cell lines. These studies have defined several important tissue-specific promoter elements including a GATA454 site (454 basepairs upstream of the transcriptional start site that involves a GATA-binding consensus sequence), a GATA54 site and an Ets35 site (that involves an Ets-binding consensus sequence). Expression studies with megakaryocytic cell lines suggest that each of these sites effects expression approximately threefold. Further, an Ets510 site was also described that had a similar effect. While these studies were underway, we pursued studies of the rat 5'-flanking region using a rat primary marrow expression system. Qualitatively, our data support the human data; however, quantitatively, we found significant differences from the human studies done in cell lines. We found that the major tissue-specific promoter element was the GATA454 site. Mutations altering this site result in an approximately fiftyfold drop in expression. In comparison, eliminating the Ets510 site by truncation or point mutation had only a twofold effect on expression. Mutations at the Ets35 site did effect expression at a high level, decreasing expression approximately fifteenfold, while mutations at the GATA54 site effected expression by approximately ninefold. In addition, using 50 bp deletions, we have preliminarily defined two domains from -450 to -351 bp and -150 to -101 bp upstream of the transcriptional start site that effected expression. The former appears to contain a positive regulatory element, while the latter appears to be a silencer element.(ABSTRACT TRUNCATED AT 400 WORDS)

An additional HpaII polymorphism in exon 2 of the human platelet membrane glycoprotein IIIa gene

A novel HpaII polymorphic site caused by a T-->G transversion at codon 40 of the GP3a locus is described. It was found together with another polymorphic HpaII site at codon 33. Both are associated with the immunologically defined HPA-1b antigen.

Rotational mobility of the fibrinogen receptor glycoprotein IIb/IIIa or integrin alpha IIb beta 3 in the plasma membrane of human platelets

Integrin alpha IIb beta 3 or glycoprotein IIb/IIIa (GPIIb/IIIa, 228 kDa) is a Ca(2+)-dependent, noncovalent heterodimer of glycoproteins IIb (GPIIb or alpha IIb, 136 kDa) and IIIa (GPIIIa or beta 3, 92 kDa), which serves as the receptor for fibrinogen and other adhesive proteins at the surface of activated platelets. We have determined the microsecond-range rotational motions of alpha IIb beta 3 in resting platelets, in isolated plasma membranes, and reconstituted in 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) bilayers. The measurements were based on the time-resolved phosphorescence anisotropy [r(t)] of erythrosin-labeled F(ab) fragments [Er-F(ab)] of monoclonal antibodies bound to alpha IIb beta 3. In general, the r(t) decays were satisfactorily fitted to the sum of the two exponential terms and a constant, from which the initial anisotropy (r(in) approximately 0.05-0.11), the short (phi 1 approximately 1-14 microseconds) and the long (phi 2 approximately 15-60 microseconds) rotational correlation times, and the limiting anisotropy (r infinity approximately 0.02-0.07) were obtained. The observed values depended on the platelet preparation, temperature, Ca2+ concentration, and the antibody used. In accordance with data on the order parameter and the viscosity of the lipid bilayer of the platelet plasma membrane, phi 2 and r infinity of the alpha IIb beta 3-Er-F(ab) complexes in the three preparations decreased with the increase of temperature, the r(t) curves being fully reversible within the interval from 5 to 35 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved DNA-based identification of fetuses at risk for HPA-1a (PlA1) neonatal alloimmune thrombocytopenia

A simple and reliable procedure, based on DNA amplification and HpaII mapping, is proposed for the identification of fetuses at risk for HPA-1a (PlA1) neonatal alloimmune thrombocytopenia which could cause life-threatening haemorrhage, even in early fetal life. This typing procedure for HPA-1 alleles should help in deciding, very early, the therapeutic management of the fetuses at risk.

Characterization of the pufferfish (Fugu) genome as a compact model vertebrate genome

Cloning and sequencing techniques now allow us to characterize genes directly instead of having to deduce their properties from their effects. This new genetics reaches its apotheosis in the plan to obtain the complete DNA sequence of the human genome, but this is far beyond the capacity of present sequencing methods. Small 'model' genomes, 'such as those of Escherichia coli (4.7 megabases (Mb) and yeast (14 Mb), or even those of Caenorhabditis elegans (100 Mb) and Drosophila (165 Mb), are better scaled to existing technology. The yeast genome will contain genes with functions common to all eukaryotic cells, and those of simple multicellular organisms may throw light on the genetic specification of more complex functions. However, vertebrates differ in their morphology and development, so the ideal model would be a vertebrate genome of minimum size and complexity but with maximum homology to the human genome. Here we report the characterization of the small genome (400 Mb) of the tetraodontoid fish, Fugu rubripes. A random sequencing approach supported by gene probing shows that the haploid genome contains 400 Mb of DNA, of which more that 90% is unique. This genome is 7.5 times smaller than the human genome and because it has a similar gene repertoire it is the best model genome for the discovery of human genes.

A review of the role of platelet membrane glycoproteins in the platelet-vessel wall interaction

This review concerns our understanding of the molecular basis of platelet function in haemostasis. In particular, we indicate how research into platelet membrane glycoprotein (GP) receptors is yielding vital information on the mechanisms of platelet adhesion and aggregation. These receptors, nearly always complexes of two or more subunits, are now known to belong to distinct gene families, some of which are unique to platelets while others are widely distributed in mammalian tissues. GP Ib-IX complexes are responsible for the high-shear-rate-dependent adhesion of platelets to von Willebrand factor (vWF) exposed within the subendothelium of damaged vessels. Other adhesion receptors include members of the VLA subclass of the integrin family: VLA-2, VLA-5 and VLA-6, which mediate platelet adhesion to collagen, fibronectin and laminin, respectively. Platelet aggregation is initiated by distinct populations of receptors specific for each physiological agonist. Many of these receptors, including the highly important and recently cloned thrombin receptor, have seven transmembrane domains and possess highly selective agonist-binding determinants. Finally, we highlight platelet aggregation and the role of GP IIb-IIIa complexes which, following platelet activation, bind fibrinogen and other adhesive proteins. The latter, through being polyvalent for GP IIb-IIIa, then form the bridges linking adjoining platelets. The 'ligand-binding pocket' of GP IIb-IIIa contains at least three sequences essential for ligand binding; fibrinogen also binds to the activated complex through identified domains, one of which, the Arg-Gly-Asp (RGD) sequence, is also found in vWF and the other adhesive proteins able to support platelet aggregation. Finally, we further describe how these, and other glycoproteins in both surface and internal membrane systems, constitute a complex receptor network capable of translocation and reorganization after platelet activation. In cardiovascular disease, platelets accumulate within arteries whose luminal surface has been modified through atherosclerosis. Recent molecular advances are yielding exciting opportunities for the development of new, and more powerful, drugs acting as specific inhibitors of thrombotic processes.

Localization of the cross-linking sites of RGD and KQAGDV peptides to the isolated fibrinogen receptor, the human platelet integrin glycoprotein IIb/IIIa. Influence of peptide length

The non-covalent and Ca(2+)-dependent heterodimer GPIIb/IIIa, formed by platelet glycoproteins IIb (GPIIb) and IIIa (GPIIIa), also known as the integrin alpha IIb beta 3, is the inducible receptor for fibrinogen and other adhesive proteins on the surface of activated platelets. A fraction of the isolated GPIIb/IIIa in solution binds RGD or KQAGDV inhibitory peptides and, upon peptide removal, apparently acquires the capacity to bind fibrinogen ('activated' GPIIb/IIIa) [Du, X., Plow, E. F., Frelinger, A. L., III, O'Toole, T. E., Loftus, J. C. & Ginsberg, M. H. (1991) Cell 65, 409-416]. Photoaffinity labelling was used here to study the ligand binding site(s) of GPIIb/IIIa in solution, for which the peptides CKRKRKRKRRGDV (alpha 1), CGRGDF (alpha 2), CYHHLGGAKQAGDV (gamma 1) and CGAKQAGDV (gamma 2) were synthesized with a photoactivable cross-linker group and a fluorescent reporter group attached to the N-terminal cysteine residue. Contrary to the situation in activated platelets, both GPIIb and GPIIIa were equally labelled by the four peptides and the cross-linking sites were localized by protein chemical analyses of the fluorescently labelled tryptic peptides of both subunits. Thus, the localization of the cross-linking sites in GPIIb varies considerably with the peptide length and is very different from that localization observed in activated platelets: alpha 2 and gamma 2 were found cross-linked to the N-terminal of both the heavy (GPIIbH 42-73) and the light (GPIIbL2 30-75) chains of GPIIb; while the longer peptides alpha 1 and gamma 1 were cross-linked to the C-terminal of GPIIbH within the 696-724 and 752-768 peptide stretches, respectively. On the other hand, the cross-linking sites of the four inhibitory peptides in GPIIIa were found mainly within the proteolysis susceptible region, between the N-terminal (GPIIIa 1-52) and the core (GPIIb 423-622) highly disulphide-bonded domains, observing that the longer the peptide the closer the cross-linking site is to the N-terminal of GPIIIa: alpha 1 at GPIIIa 63-87 and 303-350; gamma 1 at GPIIIa 9-37; alpha 2 at GPIIIa 151-191; and gamma 2 at GPIIIa 303-350. These results led us to the following conclusions. (a) The GPIIIa 100-400 region contributes to the ligand-binding domain in GPIIb/IIIa both in solution and in activated platelets.(ABSTRACT TRUNCATED AT 400 WORDS)

Leukocyte integrins: structure, function and regulation of their activity

Adhesion is a crucial requirement for the correct regulation of immune and inflammatory responses. In the immune system, leukocytes can interact with each other and with vascular endothelium as well as with extracellular matrix components, changing rapidly and transiently from circulating non-adherent to adherent states. Most of these interactions are mediated by integrins. This review will focus mainly on the structure and function of integrins expressed by leukocytes. The mechanisms for regulating the functional activity of these adhesion receptors, as well as the intracellular signals transduced through integrins, are described.

The gene organisation of the human beta 2 integrin subunit (CD18)

We have studied the gene of the human beta 2 integrin subunit (CD18) and found it to be organised into 16 exons spanning a region of about 40 kb. All exon/intron boundaries conform to the GT/AG splicing consensus. The exons coding for the cysteine-rich region, which has been postulated to consist of 3 or 4 repeating elements, are not organised correspondingly. Transcription of the gene initiates from multiple sites which may be due to the absence of an upstream TATA box. The polyadenylation site is also heterogeneous. Five different sites were identified over a stretch of 10 bases.

A large-scale procedure for the isolation of integrin GPIIb/IIIa, the human platelet fibrinogen receptor

The heterodimer GPIIb/IIIa, formed by the Ca(2+)-dependent association of glycoproteins IIb (GPIIb) and IIIa (GPIIIa), is the major integrin at the platelet surface, where it serves as the receptor for fibrinogen and other adhesive proteins and plays a central role in platelet aggregation and in platelet adhesion to the subendothelium. Here we describe a procedure for the isolation of GPIIb/IIIa using as starting material either the whole particulate fraction, obtained by differential centrifugation after hypoosmotic lysis of glycerol-loaded platelets, or any of the fractions obtained by density gradient centrifugation of the whole particulate fraction. The procedure consists simply of differential extraction with Triton X-100 of the starting particulate fraction, anion-exchange chromatography of the 4% Triton X-100 supernatant, and size-exclusion chromatography of the GPIIb/IIIa-rich fraction retained in the ion-exchange column. The use of particulate fractions instead of whole platelets as the starting material for extraction together with differential extraction with Triton X-100 (two steps that are simple and inexpensive to perform) results in the early removal of many unwanted proteins, which otherwise would have to be removed at later stages at the expense of severely impairing the final yield of GPIIb/IIIa. Pure GPIIb/IIIa is obtained with a yield of about 48%, the highest so far reported, calculated with respect to the GPIIb and GPIIIa content in the starting particulate fraction. The final product can be stored in freeze-dried form without apparent changes in its physical and chemical properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms involved in platelet activation induced by a monoclonal antibody anti glycoprotein IIb-IIIa: inositol phosphate production is not the primary event

The mechanisms involved in platelet aggregation by a monoclonal antibody (mAb) P256 specific for the GPIIb-IIIa complex was investigated following metabolic 32P labelling of platelets. When compared with thrombin, inositol phosphates (InsP) production during P256-induced activation was delayed and no apparent peak, but a small and sustained production of [32P]-Ins(1,4,5)P3 and [32P]-Ins(1,3,4,5)P4, was observed between 20 and 90 s. [32P]-Ins(1,3,4)P3 was also produced with a maximum after 90 s. Addition of the ADP scavenger creatinine phosphate/creatine phosphokinase (CP/CPK) and of the cyclooxygenase inhibitor aspirin together with P256 almost totally abolished InsP formation, whereas platelet aggregation and protein phosphorylation were partially inhibited. F(ab')2 fragments of P256 also aggregated platelets but to a smaller extent than IgG, and without any measurable InsPs. To characterize further P256-induced activation, the phosphorylation of p43, the main substrate of protein kinase C (PKC) and the phosphorylation of tyrosine protein (P-Tyr) was also studied. PKC activation was smaller with P256-IgG than with thrombin but both thrombin and P265-IgG induced a similar profile of P-Tyr involving seven major bands, whereas P256-F(ab')2 only occasionally activated PKC but always significantly phosphorylated a 64,000 molecular weight P-Tyr. The data indicate that the binding of P256 to GPIIb-IIIa, in contrast with thrombin, does not initially lead directly to the activation of the phosphoinositide phospholipase C to produce InsP's but rather involves the activation of protein kinases and also both fragments F(ab')2 and Fc play a specific role in the platelet responses to the mAb. Only the crosstalk between the two pathways evoked by F(ab')2 and Fc respectively allows the activation of all platelet activation systems.

The Molecular Genetics of Platelet Membrane Proteins and their Inherited Disorders

The platelet membrane glycoproteins fulfil a vital function in platelet adhesion and aggrkgation. The advent of recombinant DNA technology has provided a wealth of new information on the sequence, structure, expression and chromosomal location of the genes encoding these proteins and Is now permitting the detailed molecular genetic analysis of both Bernard Soulier syndrome and Glanzmann's thrombasthenia.