A beta 3 integrin mutation abolishes ligand binding and alters divalent cation-dependent conformation

The alphaMbeta2 integrin and its role in neutrophil function

Neutrophils are the first cell type to arrive at the injury sites and play a critical role in host defense, by virtue of its ability to adhere and transmigrate through endothelium, to phagocytose foreign pathogens, and to produce free oxygen radicals and proteolytic enzymes. Yet, inappropriate neutrophil activation causes tissue damage and various inflammatory diseases. These physiological and pathological functions of neutrophils depend on the engagement of certain surface receptors, especially alphaMbeta2, the major beta2 integrin receptor present on neutrophil surface. Understanding of the molecular mechanisms underlying ligand binding by alphaMbeta2, as well as the roles of alphaMbeta2-ligand interactions in neutrophil functions will enable us to regulate more precisely neutrophil activities: that is, to promote their host defense functions, and at the same time to minimize their deleterious effects on normal cells.

Peptide ligands can bind to distinct sites in integrin alphaIIbbeta3 and elicit different functional responses

The spatial relationship between the binding sites for two cyclic peptides, cyclo(S,S)KYGCRGDWPC (cRGD) and cyclo(S,S)KYGCHarGDWPC (cHarGD), high affinity analogs for the RGD and HLGGAKQAGDV peptide ligands, in integrin alphaIIbbeta3 (GPIIb-IIIa) has been characterized. For this purpose, cRGD and cHarGD were labeled with fluorescein isothiocyanate and tetramethylrhodamine 5-isothiocyanate, respectively. Both cyclic peptides were potent inhibitors of fibrinogen binding to alphaIIbbeta3, particularly in the presence of Mn2+; IC50 values for cRGD and cHarGD were 1 and <0.1 nM in the presence of Mn2+. Direct binding experiments and fluorescence resonance energy transfer analysis using the purified receptor showed that both peptides interacted simultaneously with distinct sites in alphaIIbbeta3. The distance between these sites was estimated to be 6.1 +/- 0.5 nm. Although cRGD bound preferentially to one site and cHarGD to the other, the sites were not fully specific, and each cyclic peptide or its linear counterpart could displace the other to some extent. The binding affinity of the cHarGD site was dramatically affected by Mn2+. cRGD, but not cHarGD, bound to recombinant beta3-(95-373) in a cation-dependent manner, indicating that the cRGD site is located entirely within this fragment. With intact platelets, binding of c-RGD and cHarGD to alphaIIbbeta3 resulted in distinct conformational alterations in the receptor as indicated by the differential exposure of ligand-induced binding site epitopes and also induced the opposite on membrane fluidity as shown by electron paramagnetic resonance analyses using 5-doxylstearic acid as a spin probe. These data support the concept the two peptide ligands bind to distinct sites in alphaIIbbeta3 and initiate different functional consequences within the receptor itself and within platelets.

Cellular entry of hantaviruses which cause hemorrhagic fever with renal syndrome is mediated by beta3 integrins

Hantaviruses replicate primarily in the vascular endothelium and cause two human diseases, hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). In this report, we demonstrate that the cellular entry of HFRS-associated hantaviruses is facilitated by specific integrins expressed on platelets, endothelial cells, and macrophages. Infection of human umbilical vein endothelial cells and Vero E6 cells by the HFRS-causing hantaviruses Hantaan (HTN), Seoul (SEO), and Puumala (PUU) is inhibited by antibodies to alphavbeta3 integrins and by the integrin ligand vitronectin. The cellular entry of HTN, SEO, and PUU viruses, but not the nonpathogenic Prospect Hill (PH) hantavirus (i.e., a virus with no associated human disease), was also mediated by introducting recombinant alphaIIbbeta3 or alphavbeta3 integrins into beta3-integrin-deficient CHO cells. In addition, PH infectivity was not inhibited by alphavbeta3-specific sera or vitronectin but was blocked by alpha5beta1-specific sera and the integrin ligand fibronectin. RGD tripeptides, which are required for many integrin-ligand interactions, are absent from all hantavirus G1 and G2 surface glycoproteins, and GRGDSP peptides did not inhibit hantavirus infectivity. Further, a mouse-human hybrid beta3 integrin-specific Fab fragment, c7E3 (ReoPro), also inhibited the infectivity of HTN, SEO, and PUU as well as HPS-associated hantaviruses, Sin Nombre (SN) and New York-1 (NY-1). These findings indicate that pathogenic HPS- and HFRS-causing hantaviruses enter cells via beta3 integrins, which are present on the surfaces of platelets, endothelial cells, and macrophages. Since beta3 integrins regulate vascular permeability and platelet function, these findings also correlate beta3 integrin usage with common elements of hantavirus pathogenesis.

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.

Temporary and partial inhibition of platelets by SM-20302 prevents coronary artery thrombosis in a chronic canine model

We proposed that temporary and partial platelet inhibition by a GPIIb/IIIa receptor antagonist, SM-20302, would provide sustained antithrombotic efficacy in a chronic model of coronary artery thrombosis. Instrumented, conscious dogs received vehicle (Group I, n = 7), low dose SM-20302 (30 microg/kg bolus + 1 microg/kg/min infusion for 6 h) (Group II, n = 7), or high dose SM-20302 (100 microg/kg bolus + 1 microg/kg/min infusion for 6 h) (Group III, n = 7). Thrombosis was initiated by electrolytic injury to the circumflex coronary artery. Coronary blood flow was monitored for 6 h on day 1 and days 2-6. Platelet aggregation was performed in platelet-rich plasma prepared from citrated or heparinized blood. At 6 h, both doses of SM-20302 inhibited adenosine diphosphate-induced platelet aggregation completely (> 90%) in citrated platelet-rich plasma, but incompletely (57-59%) in heparinized platelet-rich plasma. Platelet reactivity returned to baseline values at 24 h. Control animals developed thrombotic occlusion on Day 1. Both doses of SM-20302 maintained vessel patency during the infusion period (Day 1) and the subsequent 5 days. Myocardial infarct size and mortality in the drug treated groups were reduced compared to the vehicle group. Thus, temporary inhibition of platelet reactivity by SM-20302 is associated with sustained prevention of primary thrombus formation, and reduction in infarct size and mortality.

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

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

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

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

Origin of the integrin-mediated signal transduction. Functional studies with cell cultures from the sponge Suberites domuncula

Sponges (phylum Porifera) represent the phylogenetically oldest metazoan animals. Recently, from the marine sponge Geodia cydonium a first cDNA encoding a putative integrin receptor molecule was isolated. In the present study basic functional experiments have been conducted to test the hypothesis that in sponges integrin polypeptides also function as adhesion molecules and as outside-in signaling molecules. The sponge Suberites domuncula has been used for the experiments because from this sponge only has a cell culture been established. Here we report that aggregation factor (AF)-mediated cell-cell adhesion is blocked by the RGDS peptide which is known to interact with beta integrin. Both RGDS and AF were found to stimulate DNA synthesis within 24 h. The beta subunit of the integrin receptor was cloned from S. domuncula; the estimated 91-kDa molecule comprises the characteristic signatures. Evolutionary conservation of the beta integrin was assessed by comparison with corresponding beta integrin subunits from evolutionary higher metazoan taxa. Addition of RGDS or of AF to isolated cells of S. domuncula causes a rapid (within 1-2 min) increase in the intracellular Ca2+ concentration which is further augmented in the presence of Ca2+. Furthermore, incubation of the cells with RGDS or AF causes an activation of the GTP-binding protein Ras. In addition it is shown that after a prolonged incubation of the cells with RGDS and AF the expression of the genes coding for Ras and for calmodulin is upregulated. These results suggest that the integrin receptor functions in the sponge system not only as adhesion molecule but also as a molecule involved in outside-in signaling.

Functional beta1-integrins release the suppression of fibronectin matrix assembly by vitronectin

beta1-null GD25 fibroblasts adherent to vitronectin fail to bind the N-terminal 70-kDa matrix assembly domain of fibronectin or to assemble fibronectin (Sakai, T., Zhang, Q., Fässler, R., and Mosher, D. F. (1998) J. Cell Biol. 141, 527-538). We have made four observations that extend this finding. First, the presence of vitronectin on a substrate that otherwise can support fibronectin assembly has a dominant-negative effect on assembly. Second, the dominant-negative effect is lost when active beta1A is expressed. Third, beta1A containing the extracellular D130A inactivating mutation has a dominant-negative effect on fibronectin assembly. Fourth, beta1-null cells adherent to vitronectin are flat and lack filopodia, whereas beta1-null cells adherent to fibronectin or beta1A-expressing cells adherent to either vitronectin or fibronectin are contracted and exhibit numerous filopodia. These results reveal, therefore, that GD25 cells adherent to vitronectin can only assume a shape suitable for assembly of fibronectin when there is a countervailing signal from functional beta1-integrins.

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

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

Interaction of the Fungal Pathogen Candida albicans with Integrin CD11b/CD18: Recognition by the I Domain Is Modulated by the Lectin-Like Domain and the CD18 Subunit

Interactions of microorganisms with integrins are central to the host defense mechanisms. The leukocyte integrin CD11b/CD18 is the principal adhesion receptor on leukocytes for Candida albicans, a major opportunistic pathogen. In this study we have investigated the roles of three regions within the receptor, the inserted (I) and lectin-like domains within the CD11b subunit, and the CD18 subunit, in CD11b/CD18-C. albicans interactions. We report four major findings. 1) A mutation in CD18 exerts a dominant negative effect on the function of the CD11b/CD18 complex. This interpretation is based on the observation that in the absence of CD18, the CD11b subunit alone binds C. albicans well, but a single point mutation at Ser138 of CD18 abolishes CD11b/CD18 binding of the fungus. 2) The lectin-like domain is not sufficient for CD11b/CD18-C. albicans interactions. Rather, the lectin-like domain appears to influence CD11b/CD18 binding activity by modulating the function of the I domain. 3) The I domain is the primary binding site for C. albicans in the receptor and is sufficient to support an efficient interaction. 4) We have identified specific amino acid sequences within the I domain that engage the microorganism. Compared with other ligands of CD11b/CD18, C. albicans has some unique as well as common contact sites within the I domain of the receptor. Such unique contact sites may underlie the ability of C. albicans to modulate CD11b/CD18 function and raise the possibility for selective interference of the microorganism-host leukocyte interactions.

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

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

Differential effects of integrin alpha chain mutations on invasin and natural ligand interaction

To determine if recognition of the Yersinia pseudotuberculosis invasin protein and natural substrates requires identical integrin residues, a region of the human alpha3 integrin chain predicted to be involved in substrate adhesion was targeted for mutation. One point mutation located in a region of the third N-terminal repeat of the alpha3 chain, alpha3-W220A, failed to promote adhesion to the natural alpha3 beta1 substrate epiligrin but maintained near wild type levels of adhesion to invasin. A second nearby mutation, alpha3-Y218A, which showed no detectable adhesion to epiligrin, was only partially attenuated for invasin binding as well as invasin-mediated bacterial uptake. A third substitution, alpha3-D154A, predicted to be in the second N-terminal repeat not known to be implicated in cell adhesion, was competent for invasin-promoted adhesion events and appeared to encode a receptor of increased activity, as it had a higher efficiency than wild type receptor for adhesion to epiligrin. Cell lines expressing this derivative were not recognized by a function blocking anti-alpha3 antibody, indicating that the second and third repeats of the alpha3 chain are either closely linked in space or the second repeat can modulate activity of the third. Differential effects on substrate adhesion do not appear to be associated with all integrin alpha chain mutations, as alpha4 chain mutations affecting the divalent cation binding domains depressed adhesion to invasin to a significant extent.

Identification of a domain on the integrin alpha5 subunit implicated in cell spreading and signaling

The alpha5 beta1 integrin is a cell surface receptor for fibronectin implicated in several cellular activities including cell proliferation, differentiation, and migration. The primary site at which the alpha5 beta1 integrin interacts with fibronectin is the RGD (Arg-Gly-Asp) amino acid sequence. In general, the sites on the integrin alpha subunits involved in ligand binding are not well characterized. Based on previous cross-linking studies, sequence alignment, predicted conformation, and intron-exon boundaries, we identified a 144-residue region (positions 223-367) on the alpha5 subunit as a putative binding region and divided it into four subdomains named domains I, II, III, and IV. Chimeric receptors were prepared in which sequences on the alpha5 subunit were exchanged with the corresponding sequences on the alpha6 subunit, which is specific for laminin and does not bind via an RGD sequence. The mutated human alpha5 integrin gene was transfected into CHO B2 cells, which are deficient in alpha5 expression. Only chimeras of domain III or IV express on the cell surface. Both of these chimeras decreased the adhesion, spreading, focal adhesion assembly, and migration on fibronectin. The adhesion of the chimeric receptors to fibronectin remained sensitive to the RGD peptide, and antibodies that inhibit interaction with the fibronectin synergy site and RGD loop remain inhibitory for the chimeras, indicating that our chimeras do not inhibit binding to either the RGD or synergy sites. Finally, the affinity of soluble fibronectin to cells via the alpha5 beta1 receptor decreased only about 3-fold. This decrease is substantially less than the observed effects on migration and spreading, which were not altered by changes in substrate concentration. Thus, the alteration in binding sites does not easily account for the changes in cell spreading and focal adhesion assembly. The tyrosine phosphorylation and focal adhesion assembly that are seen when cells expressing the wild type alpha5 receptor adhere to fibronectin were inhibited in cells expressing the chimeric receptors. Therefore, our results suggest that the chimeras of these domains likely interrupt alpha5-mediated conformational signaling.

Adhesive and Signaling Properties of a Naturally Occurring Allele of Glycoprotein IIIa With an Amino Acid Substitution Within the Ligand Binding Domain—The Pena/PenbPlatelet Alloantigenic Epitopes

Platelet membrane glycoprotein IIIa (GPIIIa) is the most polymorphic integrin subunit in man, with at least seven recognized allelic isoforms present in the human gene pool. Whether these allelic variants of the GPIIb-IIIa complex differ in the ability to interact with the adhesive ligand fibrinogen (Fg) is still unknown. Since the Pena and Penb allelic forms of GPIIIa are distinguished by a single Arg143Gln amino acid substitution within the RGD binding domain of GPIIIa and anti-Pena human alloantibodies have been shown to bind GPIIb-IIIa on the platelet surface and inhibit ADP-induced platelet aggregation, we expressed both forms of this integrin in Chinese hamster ovary (CHO) cells and examined the relative adhesive properties. Both allelic forms of GPIIb-IIIa were expressed on the cell surface and were recognized by a well-characterized panel of murine and human monoclonal and polyclonal antibodies. Like Pena, the Penb form of GPIIb-IIIa could undergo conformational changes in response to RGD peptide binding, and could be induced by activating antibodies to bind Fg and the Fg mimetic antibody P1-55. The binding affinity for Fg of the Pena form of the GPIIb-IIIa complex was not significantly different from that of the Penb form, nor was its ability to signal to focal adhesion kinase, suggesting that Arg143Gln polymorphism has little or no effect on integrin function. Examination of the functional consequences of other integrin polymorphisms may be necessary to determine whether they constitute a risk factor for thrombosis or hemorrhage.

© 1998 by The American Society of Hematology.

Adhesive and Signaling Properties of a Naturally Occurring Allele of Glycoprotein IIIa With an Amino Acid Substitution Within the Ligand Binding Domain—The Pena/PenbPlatelet Alloantigenic Epitopes

Platelet membrane glycoprotein IIIa (GPIIIa) is the most polymorphic integrin subunit in man, with at least seven recognized allelic isoforms present in the human gene pool. Whether these allelic variants of the GPIIb-IIIa complex differ in the ability to interact with the adhesive ligand fibrinogen (Fg) is still unknown. Since the Pena and Penb allelic forms of GPIIIa are distinguished by a single Arg143Gln amino acid substitution within the RGD binding domain of GPIIIa and anti-Pena human alloantibodies have been shown to bind GPIIb-IIIa on the platelet surface and inhibit ADP-induced platelet aggregation, we expressed both forms of this integrin in Chinese hamster ovary (CHO) cells and examined the relative adhesive properties. Both allelic forms of GPIIb-IIIa were expressed on the cell surface and were recognized by a well-characterized panel of murine and human monoclonal and polyclonal antibodies. Like Pena, the Penb form of GPIIb-IIIa could undergo conformational changes in response to RGD peptide binding, and could be induced by activating antibodies to bind Fg and the Fg mimetic antibody P1-55. The binding affinity for Fg of the Pena form of the GPIIb-IIIa complex was not significantly different from that of the Penb form, nor was its ability to signal to focal adhesion kinase, suggesting that Arg143Gln polymorphism has little or no effect on integrin function. Examination of the functional consequences of other integrin polymorphisms may be necessary to determine whether they constitute a risk factor for thrombosis or hemorrhage.

© 1998 by The American Society of Hematology.

Conformational change in an anti-integrin antibody: structure of OPG2 Fab bound to a beta 3 peptide

Antibodies are important tools to explore receptor-ligand interactions. The anti-integrin antibody OPG2 binds in an RGD-related manner to the alphaIIb beta3 integrin as a molecular mimic of fibrinogen. The Fab fragment from OPG2 was cocrystallized with a peptide from the beta3 subunit of the integrin representing a site that binds RGD. The crystal structure of the complex was determined at 2.2-A resolution and compared with the unbound Fab. On binding the integrin peptide there were conformational changes in CDR3 of the heavy chain. Also, a significant shift across the intermolecular interface between the CH1-CL domains was observed so that the angle of rotation relating the two domains was reduced by 15 degrees. This unusual conformational adjustment represents the first example of ligand-induced conformational changes in the carboxyl domains of a Fab fragment.

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

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

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

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

Molecular Regulation of the Interaction Between Leukocyte Function-Associated Antigen-1 and Soluble ICAM-1 by Divalent Metal Cations

Surface plasmon resonance (SPR) was used to investigate and characterize the interaction between LFA-1 and sICAM-1 (a soluble form of ICAM-1). Full-length LFA-1 was immobilized on a hydrophobic surface, and sICAM-1 binding was monitored in a flow cell format. The binding of sICAM-1 to LFA-1 was specific and dependent upon Mg2+; Abs to both sICAM-1 and LFA-1 blocked the interaction, and EDTA abolished all binding. Association and dissociation rate constants (ka and kd, respectively) for sICAM-1 were 2.24 × 105 M−1 s−1 and 2.98 × 10−2 s−1, respectively, giving a calculated KICAM of 133 nM. Since the LFA-1/ICAM-1 interaction is highly sensitive to the presence of metal cations, SPR was also used to probe the affinity of the metal binding sites. The KMg values were 160 and 12 μM in the absence (EGTA) and the presence of Ca2+ (100 μM), respectively; in addition, KMn was 2 μM in the presence of Ca2+ (100 μM). Increasing Ca2+ into the millimolar concentration range, however, resulted in a competitive displacement of Mg2+/Mn2+ and decreased sICAM-1 binding. Based on these data, a synergistic model for the molecular regulation of LFA-1 by divalent metal cations is proposed, and implications to cellular adhesion are discussed.

beta3 Integrins mediate the cellular entry of hantaviruses that cause respiratory failure

Newly emerged hantaviruses replicate primarily in the pulmonary endothelium, cause acute platelet loss, and result in hantavirus pulmonary syndrome (HPS). We now report that specific integrins expressed on platelets and endothelial cells permit the cellular entry of HPS-associated hantaviruses. Infection with HPS-associated hantaviruses, NY-1 and Sin Nombre virus (SNV), is inhibited by antibodies to beta3 integrins and by the beta3-integrin ligand, vitronectin. In contrast, infection with the nonpathogenic (no associated human disease) Prospect Hill virus was inhibited by fibronectin and beta1-specific antibodies but not by beta3-specific antibodies or vitronectin. Transfection with recombinant alphaIIb beta3 or alphav beta3 integrins rendered cells permissive to NY-1 and SNV but not Prospect Hill virus infection, indicating that alphaIIb beta3 and alphav beta3 integrins mediate the entry of NY-1 and SNV hantaviruses. Furthermore, entry is divalent cation independent, not blocked by arginine-glycine-aspartic acid peptides and still mediated by, ligand-binding defective, alphaIIb beta3-integrin mutants. Hence, NY-1 and SNV entry is independent of beta3 integrin binding to physiologic ligands. These findings implicate integrins as cellular receptors for hantaviruses and indicate that hantavirus pathogenicity correlates with integrin usage.

Localisation of a novel adhesion blocking epitope on the human beta 1 integrin chain

Members of the beta 1 integrin family mediate cellular adherence to a wide range of extracellular and cell surface associated ligands. Conformational changes have been shown to be associated with integrin activation and ligand binding. Some studies suggest that there may be a restricted region of the beta 1 integrin that serves as the target for regulatory antibodies which can inhibit or stimulate integrin function. Here we identify an inhibitory epitope that is located at a distinct sight from that suggested for other inhibitory antibodies. Three different adhesion blocking antibodies, JB1A, C30B, and D11B bind to a peptide corresponding to residues 82-87 of the mature beta 1 chain. Mn++ inhibited the binding of JB1A to purified beta 1 integrin. In contrast the binding of several other antibodies to beta 1 were not influenced by these conditions. JB1A binding to purified peptide was also inhibited by Mn++ suggesting that it related to interference with the antibody function rather than a cation dependent change in the epitope. Our data 1) directly demonstrates the peptide sequence recognised by three adhesion blocking antibodies to the human beta 1 integrin chain 2) identifies a novel epitope located at residues 82-87, distinct from that of previously described regulatory epitopes 3) characterises a Mn++ sensitive antibody integrin interaction. Collectively, these results indicate the existence of multiple regulatory sites on the beta 1 integrin molecule.

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

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

Physical and biochemical regulation of integrin release during rear detachment of migrating cells

Cell migration can be considered as a repeated cycle of membrane protrusion and attachment, cytoskeletal contraction and rear detachment. At intermediate and high levels of cell-substratum adhesiveness, cell speed appears to be rate-limited by rear detachment, specifically by the disruption of cytoskeleton-adhesion receptor-extracellular matrix (ECM) linkages. Often, cytoskeletal linkages fracture to release integrin adhesion receptors from the cell. Cell-extracellular matrix bonds may also dissociate, allowing the integrins to remain with the cell. To investigate molecular mechanisms involved in fracturing these linkages and regulating cell speed, we have developed an experimental system to track integrins during the process of rear retraction in Chinese hamster ovary (CHO) cells. Integrin expression level was varied by transfecting CHO B2 cells, which express very little endogenous alpha5 integrin, with a plasmid containing human alpha5 integrin cDNA and sorting the cells into three populations with different alpha5 expression levels. Receptor/ligand affinity was varied using CHO cells transfected with either alphaIIbbeta3 or alphaIIbbeta3(beta1-2), a high affinity variant. alphaIIbbeta3(beta1-2) is activated to a higher affinity state with an anti-LIBS2 antibody. Fluorescent probes were conjugated to non-adhesion perturbing anti-integrin antibodies, which label integrins in CHO cells migrating on a matrix-coated glass coverslip. The rear retraction area was determined using phase contrast microscopy and integrins initially in this area were tracked by fluorescence microscopy and a cooled CCD camera. We find that rear retraction rate appears to limit cell speed at intermediate and high adhesiveness, but not at low adhesiveness. Upon rear retraction, the amount of integrin released from the cell increases as extracellular matrix concentration, receptor level and receptor-ligand affinity increase. In fact, integrin release is a constant function of cell-substratum adhesiveness and the number of cell-substratum bonds. In the adhesive regime where rear detachment limits the rate of cell migration, cell speed has an inverse relationship to the amount of integrin released at the rear of the cell. At high cell-substratum adhesiveness, calpain, a Ca2+-dependent protease, is also involved in release of cytoskeletal linkages during rear retraction. Inhibition of calpain results in decreased integrin release from the cell membrane, and consequently a decrease in cell speed, during migration. These observations suggest a model for rear retraction in which applied tension and calpain-mediated cytoskeletal linkage cleavage are required at high adhesiveness, but only applied tension is required at low adhesiveness.

Divalent cations and ligands induce conformational changes that are highly divergent among beta1 integrins

Here we show striking differences in conformational regulation among beta1 integrins. Upon manganese stimulation, a beta1 epitope defined by monoclonal antibody (mAb) 9EG7 was induced strongly (on alpha4beta1), moderately (on alpha5beta1), weakly (on alpha2beta1), or was scarcely detectable (on alpha6beta1 and alpha3beta1). Comparable results were seen for the beta1 epitope defined by mAb 15/7. Likewise, soluble ligands caused strong (alpha4beta1), moderate (alpha5beta1), weak (alpha2beta1, alpha6beta1), or minimal (alpha3beta1) induction of the 9EG7 epitope. Exchange or deletion of alpha chain cytoplasmic tails did not alter Mn2+-induced 9EG7 epitope levels. Upon removal of calcium by EGTA or EDTA, the hierarchy of 9EG7 epitope induction was similar (alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), except that EGTA reduced rather than induced 9EG7 expression on alpha4beta1. Thus in contrast to other beta1 integrins, calcium uniquely supports constitutive expression of the 9EG7 epitope on alpha4beta1. Likewise, calcium supported vascular cell adhesion molecule-stimulated 9EG7 appearance on alpha4beta1, whereas calcium inhibited ligand-induced 9EG7 epitope on other integrins. Constitutive expression of 9EG7 on alpha4beta1 was eliminated by a D698E mutation in alpha4, suggesting that Asp-698 may play a key role in maintaining atypical alpha4beta1 response to calcium. In conclusion, our results (i) demonstrate that mAb such as 9EG7 and 15/7 have limited diagnostic utility as reporters of ligand or Mn2+ occupancy for beta1 integrins, (ii) indicate pronounced differences in conformational flexibilities (alpha4beta1 > alpha5beta1 > alpha2beta1 > alpha6beta1 > alpha3beta1), (iii) allow us to hypothesize that beta1 integrins may differ markedly in conformation-dependent inside-out signaling, and (iv) have uncovered an atypical alpha4beta1 response to calcium that requires alpha4 Asp-698.

Defining extracellular integrin alpha-chain sites that affect cell adhesion and adhesion strengthening without altering soluble ligand binding

It was previously shown that mutations of integrin alpha4 chain sites, within putative EF-hand-type divalent cation-binding domains, each caused a marked reduction in alpha4beta1-dependent cell adhesion. Some reports have suggested that alpha-chain "EF-hand" sites may interact directly with ligands. However, we show here that mutations of three different alpha4 "EF-hand" sites each had no effect on binding of soluble monovalent or bivalent vascular cell adhesion molecule 1 whether measured indirectly or directly. Furthermore, these mutations had minimal effect on alpha4beta1-dependent cell tethering to vascular cell adhesion molecule 1 under shear. However, EF-hand mutants did show severe impairments in cellular resistance to detachment under shear flow. Thus, mutation of integrin alpha4 "EF-hand-like" sites may impair 1) static cell adhesion and 2) adhesion strengthening under shear flow by a mechanism that does not involve alterations of initial ligand binding.

The alphavbeta3 integrin regulates alpha5beta1-mediated cell migration toward fibronectin

This study examines the interactions of alphavbeta3 and alpha5beta1 in the regulation of cell migration. Human embryonic kidney (HEK) 293 cells that express alpha5beta1 endogenously were transfected with alphavbeta3 and beta3 mutants, and their attachment and migration to fibronectin (Fn) and vitronectin (Vn) were measured. An alphavbeta3 blocking antibody and the alphavbeta3 ligand cyclic G-Pen-GRGDSPC-A inhibited alpha5beta1-mediated migration toward Fn, but not attachment to Fn. This function was alphavbeta3-specific since alphavbeta5 transfection and alphavbeta5 blocking antibody did not produce this effect. Mutations introduced into the beta3 integrin subunit to dissect this phenomenon revealed the following. Disruption of the ligand binding domain by the Glanzmann thrombasthenia mutation beta3-D119Y constitutively abolished migration toward both Vn and Fn, and attachment to Vn but not to Fn. Insertion of the Glanzmann mutation beta3-S752P into the cytoplasmic domain or its truncation (beta3-Delta717) abolished binding to Vn but not to Fn. Inhibition of migration toward Fn was inhibited in these cells by alphavbeta3 blocking antibody. alphavbeta3-mediated inhibition was, however, abolished by truncation of the transmembrane domain (beta3-Delta693). These findings demonstrate alphavbeta3 regulation of alpha5beta1-mediated cell migration and suggest that the beta3 transmembrane domain is essential for this function.

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

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

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

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

Structural basis of integrin-mediated signal transduction

Integrins are a family of alpha/beta heterodimers of cell adhesion receptors that mediate cell-extracellular matrix and cell-cell interactions. Both alpha and beta subunits have a large extracellular domain and a short cytoplasmic domain. The alpha subunit has seven sequence repeats of 60-70 residues in its N-terminal region. The beta-propeller model, in which seven four-stranded beta-sheets are arranged in a torus around a pseudosymmetry axis, has been proposed as a structural model of these seven repeats. Several predicted loops critical for ligand binding have been identified in the upper face of the proposed beta-propeller model. Several alpha subunits (e.g., alpha 2, alpha L and alpha M) have I-domains of about 200 residues inserted between their second and third repeats. These I-domains adopt a Rossman-fold structure and have major ligand and cation binding sites (the MIDAS site) on their surfaces. The beta subunit has an I-domain-like structure in its N-terminal region. This structure includes multiple sequences/conserved oxygenated residues critical for ligand binding (e.g., Asp-119 in beta 3), and non-conserved residues critical for ligand specificities. Several "activation-dependent" epitopes have been identified in the Cys-rich (stalk) region of beta 1. It has yet to be determined how these multiple ligand binding sites in the alpha and beta subunits are involved in ligand binding, and how conformational changes on activation/ligand occupancy relate to signal transduction.

Analysis of the tetraspanin CD9-integrin alphaIIbbeta3 (GPIIb-IIIa) complex in platelet membranes and transfected cells

The platelet integrin, alphaIIbbeta3 (GPIIb-IIIa), and the tetraspanin, CD9, are integral membrane proteins that are abundant in platelet membranes. We have identified several proteins, including CD9, which were co-precipitated by anti-alphaIIbbeta3 antibody from untreated, resting platelets that were solubilized with the poly(oxyethylene) non-ionic detergent, Brij-35. Immunoblot and quantitative immunoprecipitation showed that the association of alphaIIbbeta3 with CD9 is specific and stoichiometric. The interaction between CD9 and alphaIIbbeta3 is probably hydrophobic, as Triton X-100 and hydrophobic detergents of the Brij series completely dissociated the CD9-alphaIIbbeta3 complex. Recombinant CD9 and alphaIIbbeta3 can associate after transfection into Chinese hamster ovary cells, as seen by co-immunoprecipitation and co-localization in the periphery of spreading cells and in the lamellipodia of cells plated on fibrinogen. This co-localization is absent from focal adhesions. Furthermore, anti-CD9-coated latex beads clustered alphaIIbbeta3 with CD9. This work indicates that the tetraspanin, CD9, is associated with beta3 integrins in resting platelets and transfected cells.

Allostery and proteolysis: two novel modes of regulating integrin function

Integrins are involved in transmitting signals between the cytoplasm and the extracellular matrix. Importantly, the transfer of information is bi-directional: signals flow inside-out and outside-in. Here, I discuss two potential modes by which integrin function is likely to be regulated. It is hypothesized that the integrin cytoplasmic tails are proteolytic substrates, and that cleavage of the cytoplasmic domain regulates the ligand binding affinity of integrins. It is also hypothesized that the ligand binding site is allosterically regulated by separate divalent ion binding sites that independently control ligand association and dissociation rate. Both hypotheses are suggested by reports in the literature and can be tested experimentally.

Effect of Ca2+ on GP IIb-IIIa interactions with integrilin: enhanced GP IIb-IIIa binding and inhibition of platelet aggregation by reductions in the concentration of ionized calcium in plasma anticoagulated with citrate

BACKGROUND

Integrilin (eptifibatide), a potent inhibitor of the fibrinogen binding function of GP IIb-llla, has been shown to reduce the thrombotic complications of angioplasty and of acute coronary syndromes. The present study was designed to determine whether the reduced Ca2+ concentrations in plasma anticoagulated with citrate affect Integrilin binding to GP IIb-IIIa and the ex vivo pharmacodynamic measurements for this drug.

METHODS AND RESULTS

Lower concentrations of Integrilin were found to inhibit platelet aggregation in plasma anticoagulated with citrate (for ADP, mean+/-SD IC(50)=140+/-40 nmol/L, n=6; Ca2+ =40 to 50 micromol/L) than with PPACK (IC(50)=570+/-70 nmol/L, P<.0001, n=6; Ca2+ approximately 1 mmol/L). Chelation of Ca2+ with EDTA or citrate caused a similar degree of enhancement in the inhibitory activity of Integrilin. Measurements of D3 LIBS epitope expression showed that the enhanced inhibitory activity was caused by enhanced GP IIb-IIIa occupancy by Integrilin. Citrate anticoagulation decreased the amounts of Integrilin required to inhibit the binding of PAC1, a monoclonal antibody that mimics the GP IIb-IIIa binding activity of fibrinogen. Reduced Ca2+ also increased Integrilin inhibition of the binding of biotinylated fibrinogen to purified, immobilized GP IIb-IIIa.

CONCLUSIONS

These data suggest that citrate anticoagulation removes Ca2+ from GP IIb-IIIa and enhances the apparent inhibitory activity of Integrilin. This finding indicates that the inhibitory activity of Integrilin is overestimated in blood samples collected with citrate, suggesting that it may be possible to achieve greater antithrombotic efficacy beyond that observed in clinical trials to date with Integrilin.

An RGD to LDV motif conversion within the disintegrin kistrin generates an integrin antagonist that retains potency but exhibits altered receptor specificity. Evidence for a functional equivalence of acidic integrin-binding motifs

Integrin ligands almost invariably employ a variant of either the RGD or LDV motif as a key element of their receptor recognition site. These short acidic peptide sequences collaborate with specific nonhomologous flanking residues and spatially separate "synergy" sequences to determine receptor binding specificity. Although the consensus sequences for RGD and LDV motifs are quite different, their common use suggests that they might share a critical role in receptor-ligand engagement. To date, the effects of interconversion of the two motifs within a natural protein framework have not been tested; however, in this study, we have converted the natural RGD site found in the snake venom disintegrin kistrin into an LDV motif and examined the effects of the change on the specificity of integrin recognition and on disintegrin potency. While an assessment of receptor binding using cell adhesion and purified integrin solid-phase assays demonstrated recognition of recombinant RGD kistrin by alphaVbeta3 and alpha5beta1, a series of LDV kistrin chimeras did not bind to these integrins, but instead were recognized specifically by alpha4beta1. The minimal change to elicit this distinct switch in receptor specificity was found to involve alteration of only three residues within kistrin. Alanine scanning mutagenesis was used to provide further information on the functional contribution of the three residues. More important, the LDV kistrin chimeras also retained much of the characteristic potency of RGD kistrin, indicating that the kistrin scaffold is optimized for presentation of both RGD and LDV sequences. These findings provide evidence for similarities in motif pharmacophore and reinforce the hypothesis that RGD and LDV sites have an equivalent functional role in receptor binding. They also demonstrate the potential for other disintegrin-containing proteins, perhaps from the ADAM family, to employ LDV sequences for integrin binding.

Molecular evolution of integrins: genes encoding integrin beta subunits from a coral and a sponge

The integrin family of cell surface receptors is strongly conserved in higher animals, but the evolutionary history of integrins is obscure. We have identified and sequenced cDNAs encoding integrin beta subunits from a coral (phylum Cnidaria) and a sponge (Porifera), indicating that these proteins existed in the earliest stages of metazoan evolution. The coral betaCn1 and, especially, the sponge betaPo1 sequences are the most divergent of the "beta1-class" integrins and share a number of features not found in any other vertebrate or invertebrate integrins. Perhaps the greatest difference from other beta subunits is found in the third and fourth repeats of the cysteine-rich stalk, where the generally conserved spacings between cysteines are highly variable, but not similar, in betaCn1 and betaPo1. Alternatively spliced cDNAs, containing a stop codon about midway through the full-length translated sequence, were isolated from the sponge library. These cDNAs appear to define a boundary between functional domains, as they would encode a protein that includes the globular ligand-binding head but would be missing the stalk, transmembrane, and cytoplasmic domains. These and other sequence comparisons with vertebrate integrins are discussed with respect to models of integrin structure and function.

Changing ligand specificities of alphavbeta1 and alphavbeta3 integrins by swapping a short diverse sequence of the beta subunit

Integrins mediate signal transduction through interaction with multiple cellular or extracellular matrix ligands. Integrin alphavbeta3 recognizes fibrinogen, von Willebrand factor, and vitronectin, while alphavbeta1 does not. We studied the mechanisms for defining ligand specificity of these integrins by swapping the highly diverse sequences in the I domain-like structure of the beta1 and beta3 subunits. When the sequence CTSEQNC (residues 187-193) of beta1 is replaced with the corresponding CYDMKTTC sequence of beta3, the ligand specificity of alphavbeta1 is altered. The mutant (alphavbeta1-3-1), like alphavbeta3, recognizes fibrinogen, von Willebrand factor, and vitronectin (a gain-of-function effect). The alphavbeta1-3-1 mutant is recruited to focal contacts on fibrinogen and vitronectin, suggesting that the mutant transduces intracellular signals on adhesion. The reciprocal beta3-1-3 mutation blocks binding of alphavbeta3 to these multiple ligands and to LM609, a function-blocking anti-alphavbeta3 antibody. These results suggest that the highly divergent sequence is a key determinant of integrin ligand specificity. Also, the data support a recent hypothetical model of the I domain of beta, in which the sequence is located in the ligand binding site.

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

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

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

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

Evidence that the integrin beta3 and beta5 subunits contain a metal ion-dependent adhesion site-like motif but lack an I domain

The amino-terminal domain of each integrin beta subunit is hypothesized to contain an ion binding site that is key to cell adhesion. A new hypothesis regarding the structure of this site is suggested by the crystallization of the I domains of the integrin alphaL and alphaM subunits (Lee, J.-O., Rieu, P., Arnaout, M. A., and Liddington, R. (1995) Cell 80, 631-638; Qu, A., and Leahy, D. J. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 10277-10281). In those proteins, an essential metal ion is bound by a metal ion-dependent adhesion site (MIDAS). The MIDAS is presented at the apex of a larger protein module called an I domain. The metal ligands in the MIDAS can be separated into three distantly spaced clusters of oxygenated residues. These three coordination sites also appear to exist in the integrin beta3 and beta5 subunits. Here, we examined the putative metal binding site within beta3 and beta5 using site-directed mutagenesis and ligand binding studies. We also investigated the fold of the domain containing the putative metal binding site using the PHD structural algorithm. The results of the study point to the similarity between the integrin beta subunits and the MIDAS motif at two of three key coordination points. Importantly though, the study failed to identify a residue in either beta subunit that corresponds to the second metal coordination group in the MIDAS. Moreover, structural algorithms indicate that the fold of the beta subunits is considerably different than the I domains. Thus, the integrin beta subunits appear to present a MIDAS-like motif in the context of a protein module that is structurally distinct from known I domains.

A novel peptide motif for platelet fibrinogen receptor recognition

To develop a specific antagonist of platelet alphaIIbbeta3 using small linear peptides, we synthesized a series of hexapeptides that did not have an Arg-Gly-Asp (RGD) sequence and examined their anti-platelet activity and their specificity for alphaIIbbeta3. We found a novel motif sequence, Pro-X1-X2-X3-Asp-X4, where X1 to X4 were all L-form alpha-amino acids, which specifically inhibited aggregation of human platelets at submicromolar concentrations. The Pro residue at the N terminus was essential to the anti-platelet activity, and the acetylation of the imino group of this residue also resulted in the complete loss of the activity. The results of the binding assay using purified human platelet alphaIIbbeta3 and placental alphavbeta3 and those of the cell adhesion assay suggest that this motif peptide is highly specific for platelet alphaIIbbeta3 among other integrins. Flow cytometric studies using an fluorescein isothiocyanate-labeled RGD peptide showed that this motif peptide inhibited the binding of an RGD peptide to activated platelets, suggesting that it has the same inhibitory mode as RGD peptides. Conformational analysis of this motif peptide and an RGD-containing peptide suggests that the imino group of the Pro residue may substitute for the role of the guanidino group of the Arg residue of the RGD sequence.

Integrin-ligand binding properties govern cell migration speed through cell-substratum adhesiveness

Migration of cells in higher organisms is mediated by adhesion receptors, such as integrins, that link the cell to extracellular-matrix ligands, transmitting forces and signals necessary for locomotion. Whether cells will migrate or not on a given substratum, and also their speed, depends on several variables related to integrin-ligand interactions, including ligand levels, integrin levels, and integrin-ligand binding affinities. These and other factors affect the way molecular systems integrate to effect and regulate cell migration. Here we show that changes in cell migration speed resulting from three separate variables-substratum ligand level, cell integrin expression level, and integrin-ligand binding affinity-are all quantitatively predictable through the changes they cause in a single unifying parameter: short-term cell-substratum adhesion strength. This finding is consistent with predictions of a mathematical model for cell migration. The ligand concentration promoting maximum migration speed decreases reciprocally as integrin expression increases. Increases in integrin-ligand affinity similarly result in maximal migration at reciprocally lower ligand concentrations. The maximum speed attainable, however, remains unchanged as ligand concentration, integrin expression, or integrin-ligand affinity vary, suggesting that integrin coupling with intracellular motors remains unaltered.

Interaction of beta 3 integrin-derived peptides 214-218 and 217-231 with alpha IIb beta 3 complex and with fibrinogen A alpha-chain

beta 3 integrin-derived peptides 214-218 and 217-231 have been shown previously to inhibit platelet aggregation and fibrinogen binding to platelets and to purified receptor. In this paper we study the activity of both peptides in inhibition of binding of biotinylated fibrinogen to activated platelets and to immobilized alpha IIb beta 3 receptor. We found that the mechanism of this inhibition by both peptides is different 125I-labeled 214-218 peptide binds to alpha IIb beta 3 but in contrast, 125I-labeled 217-231 peptide binds to the A alpha-chain of native and gamma' fibrinogen, as judged by the cross-linking study. In solid phase assay both purified alpha IIb beta 3 and 217-231 peptide bound extensively to native and recombinant fibrinogen, and to fibrinogen with either D574E or D97E mutations in the A alpha-chain. Binding of purified alpha IIb beta 3 to gamma' fibrinogen was markedly impaired whereas binding of 217-231 was only slightly impaired in comparison with native fibrinogen. Binding of 217-231 to fibrinogen fragment X was also reduced suggesting that sequences other than RGDS and RGDF may represent binding sites for this peptide. We hypothesize that the close vicinity of fibrinogen binding site (217-231) and of the site participating in conformational changes of the alpha IIb beta 3 receptor (214-218) may facilitate fibrinogen interaction with its receptor.

Inducible tyrosine phosphorylation of the beta3 integrin requires the alphaV integrin cytoplasmic tail

We have found that the integrin beta3 chain can be phosphorylated on tyrosine residues in K562 cells transfected with alphavbeta3. Tyrosine phosphorylation of the beta3 cytoplasmic tail is induced by adhesion to alphavbeta3-specific ligand or antibody or by incubation in manganese-containing buffer. Under the same conditions, beta5 does not become tyrosine-phosphorylated in K562 transfected with alphavbeta5. Phosphorylation of the beta3 subunit requires the simultaneous presence of the alphav subunit cytoplasmic tail, because neither the alphaIIb subunit nor a truncated alphav subunit is sufficient to permit phosphorylation of beta3 when coexpressed as a heterodimer with beta3. Finally, tyrosine phosphorylation of the beta3 cytoplasmic tail occurs on both human and murine beta3 and is inducible in the ovarian carcinoma OV10 as well, independent of expression of integrin-associated protein (CD47). Tyrosine phosphorylation of the beta3 integrin subunit facilitates association of Grb-2, an adaptor protein leading to activation of the Ras signaling pathway, and may contribute to the unique functional and signaling capabilities of alphavbeta3.