A comparative analysis of cDNA-derived sequences for rat and mouse beta 3 integrins (GPIIIA) with their human counterpart

Murine models for studying treatment, prevention and pathogenesis of FNAIT

Maternal alloimmunization to paternally inherited antigens on fetal/neonatal platelets can cause fetal/neonatal alloimmune thrombocytopenia (FNAIT) after antibody-mediated removal of platelets from the fetal circulation. The complications vary from mild bleeding symptoms to severe intracranial hemorrhage and subsequent neurological impairment or death. Studies on in vivo mechanisms are challenging to measure directly in pregnant women, rendering murine models as valuable and attractive alternatives, despite some critical differences between mice and men affecting the translational value. Here we present and discuss, the different murine models that substantially have increased our knowledge and understanding of FNAIT pathogenesis - as well as pre-clinical evaluation of therapeutic and preventive strategies.

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.

Role of molecular mimicry of hepatitis C virus protein with platelet GPIIIa in hepatitis C-related immunologic thrombocytopenia

Patients with HIV-1 immune-related thrombocytopenia (HIV-1-ITP) have a unique Ab against platelet GPIIIa49-66 capable of inducing oxidative platelet fragmentation in the absence of complement. HIV-1-seropositive drug abusers are more prone to develop immune thrombocytopenia than non-drug abusers and have a higher coinfection with hepatitis C virus (HCV) than non-drug abusers (90% vs 30%). Molecular mimicry was sought by screening a phage peptide library with anti-GPIIIa49-66 antibody as bait for peptides sharing homology sequences with HCV. Several phage peptide clones had 70% homology with HCV protein. Sera from dually infected thrombocytopenic patients with HCV and HIV-ITP reacted strongly with 4 nonconserved peptides from HCV core envelope 1. Reactivity correlated inversely with platelet count (r(2) = 0.7, P < .01). Ab raised against peptide PHC09 in GPIIIa(-/-) mice induced thrombocytopenia in wild-type mice. Affinity-purified IgG against PHC09 induced oxidative platelet fragmentation in vitro. Drug abusers dually infected with HCV and HIV-1 had a greater incidence and severity of thrombocytopenia as well as titer of anti-GPIIIa49-66/PHC09 Ab. NZB/W F1 mice injected with recombinant core envelope 1 developed Ab versus PHC09 and significantly decreased their platelet count (P < .001). Thus, HCV core envelope 1 can induce thrombocytopenia by molecular mimicry with GPIIIa49-66.

Integrin beta3 regions controlling binding of murine mAb 7E3: implications for the mechanism of integrin alphaIIbbeta3 activation

Abciximab, a derivative of the murine mAb 7E3, protects against ischemic complications of percutaneous coronary interventions by inhibiting ligand binding to the alphaIIbbeta3 receptor. In this study we identified regions on integrin beta3 that control 7E3 binding. Murine/human amino acid substitutions were created in two regions of the betaA domain that previous studies found to influence 7E3 binding: the C177-C184 loop and K125-N133. The T182N substitution and a K125Q mutation reduced 7E3 binding to human beta3 in complex with alphaIIb. The introduction of both the human C177-C184 region and human W129 into murine beta3 was necessary and sufficient to permit 7E3 binding to the human alphaIIb/murine beta3 complex. Although we cannot exclude allosteric effects, we propose that 7E3 binds between C177-C184 and W129, which are within 15 A of each other in the crystal structure and close to the beta3 metal ion-dependent adhesion site. We previously demonstrated that 7E3 binds more rapidly to activated than unactivated platelets. Because it has been proposed that alphaIIbbeta3 changes from a bent to an extended conformation upon activation, we hypothesized that 7E3 binds less well to the bent than the extended conformation. In support of this hypothesis we found that 7E3 bound less well to an alphaIIbbeta3 construct locked in a bent conformation, and unlocking the conformation restored 7E3 binding. Thus, our data are consistent with alphaIIbbeta3 existing in variably bent conformations in equilibrium with each other on unactivated platelets, and activation resulting in alphaIIbbeta3 adopting a more extended conformation.

Gene therapy for platelet disorders: studies with Glanzmann's thrombasthenia

Current research aimed at correcting platelet defects are designed to further our knowledge in the use of hematopoietic stem cells for gene therapies of hemorrhagic disorders. Information gained from these studies may be directly applicable to treatment of disorders affecting platelets (e.g. Glanzmann's thrombasthenia, Bernard Soulier syndrome, gray platelet syndrome, and von Willebrand disease) as well as other disorders affecting distinct hematopoietic cell lineages. This work specifically addresses three questions: (i) can bone marrow stem cells be given sufficient genetic information to induce abnormal megakaryocytes to synthesize transgene products that help newly formed platelets to participate in normal hemostasis? (ii) can the newly synthesized receptor be maintained as a platelet-specific protein at therapeutic levels for a reasonable period of time? and (iii) will newly expressed proteins be tolerated by the immune system or become a target for B- and T-cell mediated immunity resulting in the premature destruction and clearing of the genetically altered megakaryocytes and platelets? Answers to these questions should indicate the feasibility of targeting platelets with genetic therapies that will in turn enable better management of patients with inherited bleeding disorders. The long-range benefit of this research will be an improved understanding of the regulation of protein expression during normal megakaryocytopoiesis, and the accumulation of additional scientific knowledge about normal platelet function and the way in which platelets and other cells recognize and interact with each other.

A site involving the "hybrid" and PSI homology domains of GPIIIa (beta 3-integrin subunit) is a common target for antibodies associated with quinine-induced immune thrombocytopenia

Drug-dependent antibodies (DDAbs) can cause the precipitous destruction of platelets if a patient is exposed to the drug for which the antibodies are specific. The molecular character of the epitopes recognized is poorly understood, and the mechanism by which drugs promote tight binding of these antibodies to platelet glycoproteins without linking covalently to protein or antibody is not yet known. We studied a group of quinine-dependent antibodies that react with human glycoprotein IIIa (GPIIIa; beta3-integrin subunit) but fail to recognize rat GPIIIa, despite close homology between the 2 proteins. By characterizing reactions of these antibodies with human/rat GPIIIa chimeras and selected GPIIIa mutants, we found that each of 3 quinine-dependent antibodies requires a 17-amino acid sequence in the newly recognized "hybrid" and PSI homology domains of GPIIIa for drug-dependent binding. Disulfide bonds are required to stabilize the target epitope. Monoclonal antibody AP3, which blocks the binding of these DDAbs to GPIIIa, was found to require a more limited stretch of the same peptide for its reaction with the glycoprotein. The findings suggest this region of GPIIIa may be a favored target for quinine-dependent antibodies and may provide a basis for further studies to elucidate the molecular basis of glycoprotein-drug-antibody interaction.

A functional platelet fibrinogen receptor with a deletion in the cysteine-rich repeat region of the beta(3) integrin: the Oe(a) alloantigen in neonatal alloimmune thrombocytopenia

This report describes a new low-frequency alloantigen, Oe(a), responsible for a case of neonatal alloimmune thrombocytopenia (NAIT). In a population study none of 600 unrelated blood donors was an Oe(a) carrier. By immunochemical studies the Oe(a) antigen could be assigned to platelet glycoprotein (GP) IIIa. Sequencing of GPIIIa complementary DNA from an Oe(a) (+) individual showed deletion of a lysine residue at position 611 (DeltaLys(611)). Analysis of 20 Oe(a) (-) and 3 Oe(a) (+) individuals showed that the DeltaLys(611) form of GPIIIa was related to the phenotype. Anti-Oe(a) reacted with the DeltaLys(611), but not with the wild-type isoforms on stable transfectants expressing GPIIIa, indicating that DeltaLys(611) directly induces the expression of Oe(a) epitopes. Under nonreducing conditions the Pro(33)DeltaLys(611) variant migrated with a slightly decreased molecular weight compared to the Pro(33)Lys(611) isoform suggesting that DeltaLys(611) has an influence on the disulfide bonds of GPIIIa. The Pro(33)DeltaLys(611) GPIIIa could undergo conformational changes and bind to fibrinogen in a similar manner as the Pro(33)Lys(611) isoform. No difference was found in the tyrosine phosphorylation of pp125(FAK), suggesting that DeltaLys(611) has no effect on integrin function. In contrast to all other low-frequency antigens, the DeltaLys(611) isoform was associated with the HPA-1b, but not with the high frequency HPA-1a allele. Comparison with GPIIIa DNA from nonhuman primates indicated that the HPA-1a allele represents the ancestral form of GPIIIa. It can be assumed that the Oe(a) form did arise as a result of a mutational event from an already mutated GPIIIa allele.

IVIg inhibits reticuloendothelial system function and ameliorates murine passive-immune thrombocytopenia independent of anti-idiotype reactivity

Although the mechanism of action of intravenous immunoglobulin (IVIg) in treating antibody-dependent thrombocytopenia remains unclear, most studies have suggested that IVIg blocks the function of Fc receptors in the reticuloendothelial system (RES) and/or the protective effect may be due to the presence of variable region-reactive (anti-idiotype) antibodies within IVIg. We evaluated the effect of IVIg on platelet counts in a murine model of passively induced immune thrombocytopenia (PIT). Although IVIg was unable to neutralize the binding of two platelet-specific monoclonal antibodies to their target antigens either in vivo or in vitro, it was able to prevent PIT as well as ameliorate pre-established PIT mediated by these antibodies. IVIg adsorbed against the antibody used to induce thrombocytopenia or endogenous murine immunoglobulin also protected against PIT, indicating that antibodies with anti-idiotype activity present in IVIg are not necessary for its effective treatment of PIT. IVIg significantly blocked the ability of the RES to clear antibody-sensitized red blood cells. F(ab')2 fragments of IVIg, which are unable to block the RES but retain the idiotypic regions, were ineffective at protecting mice from PIT. Our data suggest that IVIg exerts its rapid effect by inhibiting RES function and that anti-idiotype interactions are not required.

Platelet-fibrinogen interactions

Binding of fibrinogen to GPIIb-IIIa on agonist-stimulated platelets results in platelet aggregation, presumably by crosslinking adjacent activated platelets. Although unactivated platelets express numerous copies of GPIIb-IIIa on their surface, spontaneous, and potentially deleterious, platelet aggregation is prevented by tightly regulating the fibrinogen binding activity of GPIIb-IIIa. Preliminary evidence suggests that it is the submembranous actin or actin-associated proteins that constrains GPIIb-IIIa in a low affinity state and that relief of this constraint by initiating actin filament turnover enables GPIIb-IIIa to bind fibrinogen. Two regions of the fibrinogen alpha chain that contain an RGD motif, as well as the carboxyl-terminus of the fibrinogen gamma chain, represent potential binding sites for GPIIb-IIIa in the fibrinogen molecule. However, ultrastructural studies using purified fibrinogen and GPIIb-IIIa, and studies using recombinant fibrinogen in which the RGD and relevant gamma chain motifs were mutated indicate that sequences located at the carboxyl-terminal end of the gamma chain mediates fibrinogen binding to GPIIb-IIIa. There is evidence that fibrinogen itself binds to regions in the amino terminal portions of both GPIIb and GPIIIa and that the sites interacting with the fibrinogen gamma chain and with RGD-containing peptides are spatially distinct. Nonetheless, there appears to be allosteric linkage between these sites, accounting for the ability of RGD-containing peptides to inhibit platelet aggregation and arterial thrombosis.

Cloning and characterization of the murine beta(3) integrin gene promoter: identification of an interleukin-4 responsive element and regulation by STAT-6

Expression of the alpha(v)beta(3) integrin by murine bone marrow macrophages is regulated by cytokines such as IL-4 and GM-CSF through transcriptional activation of the beta(3) subunit gene. To characterize the molecular mechanisms by which such regulation occurs, we isolated the murine beta(3) integrin promoter. To this end, we first cloned a full length beta(3) cDNA and used the 5'UTR and leader peptide coding sequence to identify genomic clones containing the beta(3) promoter region. The transcriptional start site, identified by primer extension and S1 nuclease assay, is 34 nt upstream of the translation initiation codon. A 1.1 kb fragment of the promoter region drives IL-4 responsive transcription in transiently transfected murine bone marrow macrophages. Deletion analysis of the beta(3) promoter indicates the IL-4 responsive element lies between -465 to -678 nt relative to the transcriptional start site. This promoter fragment contains two overlapping STAT consensus recognition sites and nuclear extracts from BMMs contain an IL-4-inducible DNA binding factor, identified by super shift analysis, as STAT-6. Furthermore, an oligonucleotide which includes the two STAT recognition sites residing in the IL-4 responsive region of the beta(3) promoter, competes for STAT-6 binding. Confirming IL-4 induction of the integrin subunit is specifically mediated by STAT-6, beta(3) mRNA is not enhanced in BMMs derived from STAT-6 deleted mice, which however, retain their capacity to respond to GM-CSF.

High-efficiency utilization of the bovine integrin alpha(v)beta(3) as a receptor for foot-and-mouth disease virus is dependent on the bovine beta(3) subunit

We have previously reported that Foot-and-mouth disease virus (FMDV), which is virulent for cattle and swine, can utilize the integrin alpha(v)beta(3) as a receptor on cultured cells. Since those studies were performed with the human integrin, we have molecularly cloned the bovine homolog of the integrin alpha(v)beta(3) and have compared the two receptors for utilization by FMDV. Both the alpha(v) and beta(3) subunits of the bovine integrin have high degrees of amino acid sequence similarity to their corresponding human subunits in the ectodomains (96%) and essentially identical transmembrane and cytoplasmic domains. Within the putative ligand-binding domains, the bovine and human alpha(v) subunits have a 98.8% amino acid sequence similarity while there is only a 93% similarity between the beta(3) subunits of these two species. COS cell cultures, which are not susceptible to FMDV infection, become susceptible if cotransfected with alpha(v) and beta(3) subunit cDNAs from a bovine or human source. Cultures cotransfected with the bovine alpha(v)beta(3) subunit cDNAs and infected with FMDV synthesize greater amounts of viral proteins than do infected cultures cotransfected with the human integrin subunits. Cells cotransfected with a bovine alpha(v) subunit and a human beta(3) subunit synthesize viral proteins at levels equivalent to those in cells expressing both human subunits. However, cells cotransfected with the human alpha(v) and the bovine beta(3) subunits synthesize amounts of viral proteins equivalent to those in cells expressing both bovine subunits, indicating that the bovine beta(3) subunit is responsible for the increased effectiveness of this receptor. By engineering chimeric bovine-human beta(3) subunits, we have shown that this increase in receptor efficiency is due to sequences encoding the C-terminal one-third of the subunit ectodomain, which contains a highly structured cysteine-rich repeat region. We postulate that amino acid sequence differences within this region may be responsible for structural differences between the human and bovine beta(3) subunit, leading to more efficient utilization of the bovine receptor by this bovine pathogen.

Characterization of the Murine Platelet IIb Gene and Encoded cDNA

The IIb/β3 receptor is central to platelet aggregation. Biological studies of this receptor have been limited by the inability to reproduce IIb/β3 function in a cell system. Increasingly, efforts are being directed at studies of this receptor in mice models. The structure of murine (m) β3 has been reported. We now have sequenced the mIIb gene and found that it has the same size and organization as the human gene. The exon/intron borders are reported here, as are the distances between exons. mIIb protein is 1,033 amino acids (aa), 7 and 5 aa shorter than human (h) and rodent (r) IIb, respectively, with 79% and 90% homology, respectively. As part of the comparative analysis of the 3 known IIb chains included in this report, we found that a particular region of the IIb N-terminal β-propeller is highly conserved and speculate that it directly participates in ligand binding.

Characterization of the Murine Platelet IIb Gene and Encoded cDNA

The IIb/β3 receptor is central to platelet aggregation. Biological studies of this receptor have been limited by the inability to reproduce IIb/β3 function in a cell system. Increasingly, efforts are being directed at studies of this receptor in mice models. The structure of murine (m) β3 has been reported. We now have sequenced the mIIb gene and found that it has the same size and organization as the human gene. The exon/intron borders are reported here, as are the distances between exons. mIIb protein is 1,033 amino acids (aa), 7 and 5 aa shorter than human (h) and rodent (r) IIb, respectively, with 79% and 90% homology, respectively. As part of the comparative analysis of the 3 known IIb chains included in this report, we found that a particular region of the IIb N-terminal β-propeller is highly conserved and speculate that it directly participates in ligand binding.

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.

Acute Systemic Reaction and Lung Alterations Induced by an Antiplatelet Integrin gpIIb/IIIa Antibody in Mice

Shock is frequently accompanied by thrombocytopenia. To investigate the pathogenic role of platelets in shock, we examined the in vivo effects of monoclonal antibodies (MoAbs) against mouse platelet membrane proteins. Injection of the platelet-specific MoAb MWReg30 to the fibrinogen receptor (gpIIb/IIIa) rendered mice severely hypothermic within minutes. Isotype-matched control antibodies, even if they also recognized platelet surface antigens, did not induce comparable signs. MWReg30 induced early signs of acute lung injury with increased cellularity in the lung interstitium and rapid engorgement of alveolar septal vessels. Despite this in vivo activity, MWReg30 inhibited rather than stimulated platelet aggregation in vitro. MWReg30-binding to platelets led to phosphorylation of gpIIIa, but did not induce morphological signs of platelet activation. The MWReg30-induced reaction was abolished after treatment with MoAbs 2.4G2 to FcγRII/III and was absent in FcγRIII-deficient mice, clearly demonstrating the requirement for FcγRIII on involved leukocytes. Simultaneous administration of tumor necrosis factor exacerbated, whereas a tolerizing regimen of tumor necrosis factor or bacterial lipopolysaccharide completely prevented the reaction. These data suggest that platelet surface-deposited MWReg30-immune complexes lead to an acute Fc-mediated reaction with pulmonary congestion and life-threatening potential that could serve as an in vivo model of acute lung injury.

Acute Systemic Reaction and Lung Alterations Induced by an Antiplatelet Integrin gpIIb/IIIa Antibody in Mice

Shock is frequently accompanied by thrombocytopenia. To investigate the pathogenic role of platelets in shock, we examined the in vivo effects of monoclonal antibodies (MoAbs) against mouse platelet membrane proteins. Injection of the platelet-specific MoAb MWReg30 to the fibrinogen receptor (gpIIb/IIIa) rendered mice severely hypothermic within minutes. Isotype-matched control antibodies, even if they also recognized platelet surface antigens, did not induce comparable signs. MWReg30 induced early signs of acute lung injury with increased cellularity in the lung interstitium and rapid engorgement of alveolar septal vessels. Despite this in vivo activity, MWReg30 inhibited rather than stimulated platelet aggregation in vitro. MWReg30-binding to platelets led to phosphorylation of gpIIIa, but did not induce morphological signs of platelet activation. The MWReg30-induced reaction was abolished after treatment with MoAbs 2.4G2 to FcγRII/III and was absent in FcγRIII-deficient mice, clearly demonstrating the requirement for FcγRIII on involved leukocytes. Simultaneous administration of tumor necrosis factor exacerbated, whereas a tolerizing regimen of tumor necrosis factor or bacterial lipopolysaccharide completely prevented the reaction. These data suggest that platelet surface-deposited MWReg30-immune complexes lead to an acute Fc-mediated reaction with pulmonary congestion and life-threatening potential that could serve as an in vivo model of acute lung injury.

Construction of a Human Platelet Alloantigen-1a Epitope(s) Within Murine Glycoprotein IIIa: Identification of Residues Critical to the Conformation of the Antibody Binding Site(s)

The human platelet alloantigen 1 system (HPA-1) is determined by a polymorphism at position 33 in the N-terminus of human glycoprotein IIIa (GPIIIa). This naturally occurring substitution creates a conformation in the HPA-1a allelic form that can be antigenic when presented to an individual expressing the HPA-1b form. Anti–HPA-1a antibodies generated by this immune response can lead to the destruction of platelets, as seen in the clinical disorders, neonatal alloimmune thrombocytopenia (NAIT) and posttransfusion purpura (PTP). To understand better the structural requirements for recognition by these pathogenic antibodies, we investigated the N-terminal 66 amino acids from the HPA-1a form of human GPIIIa and the analogous amino acids from the nonimmunogenic murine homolog. Our objectives were to define further the boundaries of the HPA-1a epitope(s) in the N-terminus of human GPIIIa, to isolate the murine 5’ nucleotide sequence and compare the deduced murine N-terminal sequence to that of human, and to mutate the murine sequence systematically to include an HPA-1a epitope(s). Murine amino acids that differed from human were changed by site-directed mutagenesis to the analogous residues in the HPA-1a form of human GPIIIa, starting and radiating from murine position 33 (site of human polymorphism). This systematic approach allowed us to pinpoint amino acids critical to a conformation recognized by anti–HPA-1a antibodies. Our results show that an HPA-1a epitope can be created within the N-terminus of murine GPIIIa and raise the possibility that murine models of HPA-1a sensitization can be developed.

Construction of a Human Platelet Alloantigen-1a Epitope(s) Within Murine Glycoprotein IIIa: Identification of Residues Critical to the Conformation of the Antibody Binding Site(s)

The human platelet alloantigen 1 system (HPA-1) is determined by a polymorphism at position 33 in the N-terminus of human glycoprotein IIIa (GPIIIa). This naturally occurring substitution creates a conformation in the HPA-1a allelic form that can be antigenic when presented to an individual expressing the HPA-1b form. Anti–HPA-1a antibodies generated by this immune response can lead to the destruction of platelets, as seen in the clinical disorders, neonatal alloimmune thrombocytopenia (NAIT) and posttransfusion purpura (PTP). To understand better the structural requirements for recognition by these pathogenic antibodies, we investigated the N-terminal 66 amino acids from the HPA-1a form of human GPIIIa and the analogous amino acids from the nonimmunogenic murine homolog. Our objectives were to define further the boundaries of the HPA-1a epitope(s) in the N-terminus of human GPIIIa, to isolate the murine 5’ nucleotide sequence and compare the deduced murine N-terminal sequence to that of human, and to mutate the murine sequence systematically to include an HPA-1a epitope(s). Murine amino acids that differed from human were changed by site-directed mutagenesis to the analogous residues in the HPA-1a form of human GPIIIa, starting and radiating from murine position 33 (site of human polymorphism). This systematic approach allowed us to pinpoint amino acids critical to a conformation recognized by anti–HPA-1a antibodies. Our results show that an HPA-1a epitope can be created within the N-terminus of murine GPIIIa and raise the possibility that murine models of HPA-1a sensitization can be developed.

A Mutation in the Extracellular Cysteine-Rich Repeat Region of the β3 Subunit Activates Integrins IIbβ3 and Vβ3

Inside-out signaling regulates the ligand-binding function of integrins through changes in receptor affinity and/or avidity. For example, IIbβ3 is in a low-affinity/avidity state in resting platelets, and activation of the receptor by platelet agonists enables fibrinogen to bind. In addition, certain mutations and truncations of the integrin cytoplasmic tails are associated with a high-affinity/avidity receptor. To further evaluate the structural basis of integrin activation, stable Chinese hamster ovary (CHO) cell transfectants were screened for high-affinity/avidity variants of IIbβ3. One clone (AM-1) expressed constitutively active IIbβ3, as evidenced by (1) binding of soluble fibrinogen and PAC1, a ligand-mimetic antiIIbβ3antibody; and (2) fibrinogen-dependent cell aggregation. Sequence analysis and mutant expression in 293 cells proved that a single amino acid substitution in the cysteine-rich, extracellular portion of β3(T562N) was responsible for receptor activation. In fact, T562N also activated Vβ3, leading to spontaneous binding of soluble fibrinogen to 293 cells. In contrast, neither T562A nor T562Q activated IIbβ3, suggesting that acquisition of asparagine at residue 562 was the relevant variable. T562N also led to aberrant glycosylation of β3, but this was not responsible for the receptor activation. The binding of soluble fibrinogen to IIbβ3(T562N) was not sufficient to trigger tyrosine phosphorylation of pp125FAK, indicating that additional post-ligand binding events are required to activate this protein tyrosine kinase during integrin signaling. These studies have uncovered a novel gain-of-function mutation in a region of β3 intermediate between the ligand-binding region and the cytoplasmic tail, and they suggest that this region is involved in integrin structural changes during inside-out signaling.

A Mutation in the Extracellular Cysteine-Rich Repeat Region of the β3 Subunit Activates Integrins IIbβ3 and Vβ3

Inside-out signaling regulates the ligand-binding function of integrins through changes in receptor affinity and/or avidity. For example, IIbβ3 is in a low-affinity/avidity state in resting platelets, and activation of the receptor by platelet agonists enables fibrinogen to bind. In addition, certain mutations and truncations of the integrin cytoplasmic tails are associated with a high-affinity/avidity receptor. To further evaluate the structural basis of integrin activation, stable Chinese hamster ovary (CHO) cell transfectants were screened for high-affinity/avidity variants of IIbβ3. One clone (AM-1) expressed constitutively active IIbβ3, as evidenced by (1) binding of soluble fibrinogen and PAC1, a ligand-mimetic antiIIbβ3antibody; and (2) fibrinogen-dependent cell aggregation. Sequence analysis and mutant expression in 293 cells proved that a single amino acid substitution in the cysteine-rich, extracellular portion of β3(T562N) was responsible for receptor activation. In fact, T562N also activated Vβ3, leading to spontaneous binding of soluble fibrinogen to 293 cells. In contrast, neither T562A nor T562Q activated IIbβ3, suggesting that acquisition of asparagine at residue 562 was the relevant variable. T562N also led to aberrant glycosylation of β3, but this was not responsible for the receptor activation. The binding of soluble fibrinogen to IIbβ3(T562N) was not sufficient to trigger tyrosine phosphorylation of pp125FAK, indicating that additional post-ligand binding events are required to activate this protein tyrosine kinase during integrin signaling. These studies have uncovered a novel gain-of-function mutation in a region of β3 intermediate between the ligand-binding region and the cytoplasmic tail, and they suggest that this region is involved in integrin structural changes during inside-out signaling.

Three novel integrin beta3 subunit missense mutations (H280P, C560F, and G579S) in thrombasthenia, including one (H280P) prevalent in Japanese patients

We analyzed three unrelated Japanese patients with type II Glanzmann thrombasthenia (GT) for associated mutations. Polymerase chain reaction and subsequent direct sequencing of platelet RNA and genomic DNA revealed three single nucleotide substitutions of the integrin beta3 subunit gene (His (CAT)-280 to Pro (CCT), Cys (TGT)-560 to Phe (TTT), and Gly(GGC)-579 to Ser(AGC)). Interestingly, the three unrelated patients all had the H280P mutation; one was homozygous and the other two heterozygous for this mutation. Ectopic expression of wild type and mutant complexes in Chinese hamster ovary cells revealed decreased surface expression of the mutated alphaIIbbeta3 complexes, thus demonstrating that these mutations may result in the mild GT phenotypes. The identification of three unrelated patients having the same mutation (H280P) suggests that this mutation might be prevalent in the Japanese thrombasthenic population.

Mouse Cell Surface Antigens: Nomenclature and Immunophenotyping

This paper reviews cell surface Ags expressed on mouse hemopoietic and nonhemopoietic cells. The review will cover molecules included in the cluster of differentiation (CD) from CD1 to CD166 and lymphocyte Ag (Ly) series from Ly-1 to Ly-81 as well as some new Ags without current CD or Ly assignments. In addition to an update on mouse nomenclature, there will be a discussion of some known functions of the molecules and brief comments on the use of particular Ags for immunophenotyping of cell subsets. Several novel markers mentioned may prove useful in mouse immunology research.

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.

GPIIIa-(49-66) is a major pathophysiologically relevant antigenic determinant for anti-platelet GPIIIa of HIV-1-related immunologic thrombocytopenia

High-affinity (Kd = 1 x 10(-9) M) anti-platelet GPIIIa has been isolated from serum immune complexes of immunologic thrombocytopenic HIV-1-infected patients (HIV-1-ITP). Affinity-purified anti-platelet antibody reacted with a recombinant GPIIIa-(1-200) and -(1-66) fusion peptide and with an 18-mer GPIIIa-(49-66) peptide but not with seven other GPIIIa peptides spanning the length of GPIIIa. Most of the anti-platelet antibody ( approximately 85%) could be adsorbed to and eluted from a GPIIIa-(49-66) affinity column. Binding of antibody to platelets could be inhibited by GPIIIa-(49-66) or an equimolar peptide-albumin conjugate (IC50 = 2 microM). Sera from 7 control subjects and 10 classic autoimmune thrombocytopenic patients gave background reactivity with GPIIIa-(49-66). HIV-1-ITP sera from 16 patients reacted with a mean OD 6-fold greater than background (range, 4- to 9-fold). Serum anti-GPIIIa-(49-66) concentration correlated inversely with platelet count, R2 = 0.51, n = 31, P < 0. 0001. Because mouse platelet GPIIIa-(49-66) has 83% homology with human GPIIIa and mouse monocytes contain Fc receptors for the human IgG1-kappa/lambda antibody, we determined the in vivo effect of human anti-GPIIIa on mouse platelets. Affinity-purified antibody, 25-50 microg given i.p., resulted in a precipitous drop in platelet count to 30% of baseline, with nadir at 4 hr and return to normal in 36 hr. No effect was noted with control IgG. Acute thrombocytopenia could be prevented or reversed by the injection of the GPIIIa-(49-66) albumin conjugate at zero time or 2 hr after antibody, respectively, but not with a scrambled peptide-albumin conjugate. Thus HIV-1-ITP patients have high-affinity anti-platelet GPIIIa against a major antigenic determinant, GPIIIa-(49-66), which correlates inversely with platelet count and induces thrombocytopenia in mice.

Molecular requirements for assembly and function of a minimized human integrin alphaIIbbeta3

Integrin subunit compatibility within and between species plays a major role in heterodimer assembly and ligand specificity. As an example, human alphaIIb pairs only with human beta3 and does not assemble a heterodimer with beta3 from other species. We use interspecies subunit chimeras to identify molecular requirements for subunit compatibility and show that species-restricted heterodimer assembly depends on a unique hexapeptide VGSDNH in an extended loop of the hypothetical human beta3 MIDAS domain. This allows us to express alphaIIb(1-233) and beta3(111-318) as a soluble, mini-integrin that retains RGD-dependent ligand recognition. Thus, in the case of one integrin, alphaIIbbeta3, the molecular requirements for integrin subunit compatibility and ligand recognition are intimately related.

Agonist-stimulated ligand binding by the platelet integrin alpha IIb beta 3 in a lymphocyte expression system

The ligand binding activity of the platelet integrin alpha IIb beta 3 is initiated by agonist-generated intraplatelet signals. We studied this process in vitro by expressing recombinant alpha IIb beta 3 in Epstein-Barr virus-immortalized B lymphocytes. We found that phorbol ester stimulation induced the adhesion of lymphocytes expressing alpha IIb beta 3 to immobilized fibrinogen. Moreover, replacement of the transmembrane and cytoplasmic domains of the alpha and beta subunits of alpha IIb beta 3 with those of alpha L beta 2 significantly increased adherence, whereas replacement of only the cytoplasmic domains significantly decreased adherence. This suggests that transmembrane segments are involved in the agonist-induced modulation of alpha IIb beta 3 activity. Similar results were seen when the alpha IIb beta 3 activation-dependent monoclonal antibody PAC-1 was substituted for immobilized fibrinogen. We also found that the adherence of lymphocytes expressing beta 3 with either of the two alpha IIb/alpha L chimeras was similar to that of cells expressing alpha IIb beta 3, whereas the adherence of cells expressing alpha IIb with either of the two beta 3/beta 2 chimeras was substantially decreased, suggesting that the identity of the cytoplasmic domain of beta 3, but not of alpha IIb, is critical for alpha IIb beta 3 function. This report indicates that B lymphocytes contain signal transduction pathways involving protein kinase C that can increase the ligand binding activity of alpha IIb beta 3 and demonstrates the utility of these cells as an expression system for the study of agonist-stimulated alpha IIb beta 3 function.

Interleukin-4 induces expression of the integrin alpha v beta 3 via transactivation of the beta 3 gene

Osteoclastic bone resorption is dependent upon cell-matrix recognition. This process is mediated by the integrin alpha v beta 3 whose expression is enhanced, in avian osteoclast precursors, by bone-seeking steroids. The purpose of this study was to determine if bone-modulating cytokines impact on alpha v beta 3 expression by mouse marrow macrophages (BMMs), known to differentiate into osteoclasts. Of the cytokines tested. Interleukin-4 (IL-4) is most effective in increasing beta 3 mRNA levels by a mechanism involving transactivation of the beta 3 gene. Moreover, IL-4 augmented beta 3 mRNA is mirrored by plasma membrane appearance of alpha v beta 3. As IL-4 induces beta 3 and not alpha v mRNA, the beta 3 chain appears to regulate surface expression of the heterodimer. The functional significance of IL-4-induced alpha v beta 3 is underscored by the fact that, while attachment to fibronectin is unaltered, treatment of BMMs with the cytokine enhances alpha v beta 3-mediated binding to vitronectin 5-fold. Expression of this heterodimer by BMMs driven along a non-osteoclastic lineage suggests alpha v beta 3 may play a role in the inflammatory response of macrophages.

A new alternative transcript encodes a 60 kDa truncated form of integrin beta 3

A cDNA for integrin beta 3 isolated from a human erythroleukaemia (HEL) cell library contained a 340 bp insert at position 1281. This mRNA, termed beta 3c, results from the use of a cryptic AG donor splice site in intron 8 of the beta 3 gene, and is different from a previously described alternative beta 3 mRNA. The predicted open reading frame of beta 3C stops at a TAG stop codon 69 bp downstream from position 1281. It starts with the signal peptide and the 404 N-terminal extracellular residues of beta 3, encompassing the ligand binding sites, followed by 23 C-terminal intron-derived residues, corresponding to a truncated form of beta 3 lacking the cysteine-rich, transmembrane and cytoplasmic domains. Expression of beta 3C mRNA was demonstrated in human platelets, megakaryocytes, endothelial cells and HEL cells by reverse transcriptase/PCR. The beta 3C transcript was also demonstrated in the mouse, suggesting its conservation through evolution. Finally, a 60 kDa polypeptide corresponding to the beta 3C alternative transcript was demonstrated in platelets by Western blotting using a polyclonal antibody raised against a synthetic peptide designed from the beta 3C intronic sequence. Taken together, these results suggest a biological role for beta 3C, the first alternative transcript showing an altered extracellular domain of a beta integrin.