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Prepublished online as a Blood First Edition Paper on December 27, 2002; DOI 10.1182/blood-2002-07-2144. This Article Abstract Full Text (PDF) All Versions of this Article: 2002-07-2144v1 101/9/3485    most recent Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Rights and Permissions Citing Articles Citing Articles via HighWire Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Kiyoi, T. Articles by Matsuzawa, Y. Search for Related Content PubMed PubMed Citation Articles by Kiyoi, T. Articles by Matsuzawa, Y. Related Collections Cell Adhesion and Motility Signal Transduction Hemostasis, Thrombosis, and Vascular Biology Social Bookmarking                   What's this?  Previous Article  |  Table of Contents  |  Next Article  Blood, 1 May 2003, Vol. 101, No. 9, pp. 3485-3491 HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY A naturally occurring Tyr143HisIIb mutation abolishes IIb3 function for soluble ligands but retains its ability for mediating cell adhesion and clot retraction: comparison with other mutations causing ligand-binding defects Teruo Kiyoi, Yoshiaki Tomiyama, Shigenori Honda, Seiji Tadokoro, Morio Arai, Hirokazu Kashiwagi, Satoru Kosugi, Hisashi Kato, Yoshiyuki Kurata, and Yuji Matsuzawa From the Department of Internal Medicine and Molecular Science, Graduate School of Medicine, Osaka University; the Department of Blood Transfusion, Osaka University Hospital; and the Department of Laboratory Medicine, Tokyo Medical University, Japan.     Abstract Top Abstract Introduction Patients, materials, and... Results Discussion References The molecular basis for the interaction between a prototypic non-I-domain integrin, IIb3, and its ligands remains to be determined. In this study, we have characterized a novel missense mutation (Tyr143His) in IIb associated with a variant of Glanzmann thrombasthenia. Osaka-12 platelets expressed a substantial amount of IIb3 (36%-41% of control) but failed to bind soluble ligands, including a high-affinity IIb3-specific peptidomimetic antagonist. Sequence analysis revealed that Osaka-12 is a compound heterozygote for a single 521T>C substitution leading to a Tyr143His substitution in IIb and for the null expression of IIb mRNA from the maternal allele. Given that Tyr143 is located in the W3 4-1 loop of the -propeller domain of IIb, we examined the effects of Tyr143His or Tyr143Ala substitution on the expression and function of IIb3 and compared them with KO (Arg-Thr insertion between 160 and 161 residues of IIb) and with the Asp163Ala mutation located in the same loop by using 293 cells. Each of them abolished the binding function of IIb3 for soluble ligands without disturbing IIb3 expression. Because immobilized fibrinogen and fibrin are higher affinity/avidity ligands for IIb3, we performed cell adhesion and clot retraction assays. In sharp contrast to KO mutation and Asp163AlaIIb3, Tyr143HisIIb3-expressing cells still had some ability for cell adhesion and clot retraction. Thus, the functional defect induced by Tyr143HisIIb is likely caused by its allosteric effect rather than by a defect in the ligand-binding site itself. These detailed structure-function analyses provide better understanding of the ligand-binding sites in integrins. (Blood. 2003;101:3485-3491) © 2003 by The American Society of Hematology.     Introduction Top Abstract Introduction Patients, materials, and... Results Discussion References Integrins are a family of noncovalently associated heterodimeric adhesion receptors that mediate cellular attachment to the extracellular matrix and cell-cell cohesion.1,2 Integrins are involved in a variety of physiological processes, including development, immune response, wound healing, and hemostasis.1 They are also involved in pathologic processes, such as tumor metastasis, thrombosis, and athelosclerosis.1 Although its expression is restricted to the megakaryocyte/platelet lineage, IIb3 (GPIIb-IIIa) is a prototypic integrin that functions as a physiologic receptor for fibrinogen and von Willebrand factor and that plays a crucial role in platelet aggregation, normal hemostasis, and pathologic thrombosis.3 Indeed, Glanzmann thrombasthenia (GT) is an autosomal recessive bleeding disorder caused by a defect in the expression or the function of integrin IIb3.4 Recent clinical studies have shown the beneficial effects of IIb3 antagonists in patients undergoing coronary angioplasty and in those with unstable angina.5,6 Integrin  subunits are grouped into 2 classes based on the presence or absence of an inserted domain of approximately 200 amino acid residues (I- or A-domain); IIb and v subunits do not have the I-domain.7-9 Recently, the crystal structures of the extracellular segment of the other 3 integrin, v3in the presence or absence of its small ligandhave been described.10,11 As predicted, the putative ligand-binding head is primarily formed by a 7-bladed -propeller domain from v and a  I-domain from 3.10-12 The -propeller domain contains 7 4-stranded -sheets (W1-W7) arranged in a torus around a pseudosymmetry axis. However, the ligand-binding sites in the -propeller domain of the IIb subunit remain to be determined. The characterization of molecular defects in GT from dysfunctional IIb3 (variant GT) has succeeded in pinpointing ligand-binding sites and functionally important domains.13-15 We have demonstrated that 2-amino acid insertion (Arg-Thr) between amino acid residues 160 and 161 in IIb is responsible for the ligand-binding defect in a Japanese variant GT known as KO.16 The insertion is located within the small loop (Cys146-Cys167) between W2 and W3 (W3 4-1 loop) located on the upper face of the -propeller. Alanine substitutions further indicate that Asp163 within the Cys146-Cys167 loop is one of the critical residues for ligand binding.16 In this context, 2 other naturally occurring missense mutations, Pro145Ala and Leu183Pro, which impair IIb3 expression and its ligand-binding function, have been identified in the W3 4-1 loop and the W3 2-3 loop of the -propeller, respectively.17,18 In this study we demonstrate that a novel naturally occurring missense mutation (Tyr143His) within the W3 4-1 loop in IIb is responsible for a binding defect in IIb3 for soluble ligands. Because Tyr143His, KO, and Asp163Ala mutations are located within the same loop in IIb, we further compared functions of these mutant IIb3 by using cell adhesion and clot retraction assays. Compared with the KO mutation and Asp163AlaIIb3, Tyr143HisIIb3-expressing cells still had some ability for cell adhesion and clot retraction. Our results indicate that the KO mutant and Asp163AlaIIb3 impair ligand-binding function in IIb3 more severely than Tyr143HisIIb3.     Patients, materials, and methods Top Abstract Introduction Patients, materials, and... Results Discussion References Patient history Patient Osaka-12, a product of nonconsanguineous parents, was a 21-year-old Japanese woman who had a history of moderate mucocutaneous bleeding in her childhood. Hematologic examinations revealed a prolonged bleeding time (more than 15 minutes) and an absence of platelet aggregation in response to adenosine diphosphate (ADP), epinephrine, and collagen but a normal response to ristocetin. Clot retraction using the MacFarlane method was 46% (normal range, 40%-70%).19 Informed consent for analyzing their molecular genetic abnormalities was obtained from Osaka- 12 and her parents. Antibodies and synthetic ligands AP1 (GPIb-specific monoclonal antibody [mAb]), AP2 (IIb3-specific mAb), and AP5 (3-specific mAb) were generously provided by Dr Thomas J. Kunicki (Scripps Research Institute, La Jolla, CA).20,21 AP3 (3-specific mAb) was a generous gift from Dr Peter J. Newman (Blood Center of Southeastern Wisconsin, Milwaukee).22 OP-G2 is a ligand-mimetic IIb3-specific mAb that binds to nonactivated and activated IIb3.23 PAC-1, a ligand-mimetic IIb3-specific mAb that binds specifically to activated IIb3, was kindly provided by Dr Sanford J. Shattil (Scripps Research Institute).24 PT25-2 (IIb3-specific mAb) activates IIb3 and was a kind gift from Drs Makoto Handa and Yasuo Ikeda (Keio University, Tokyo, Japan).25 PMI-1 (IIb-specific mAb), anti-LIBS1 (3-specific mAb), and anti-LIBS6 (3-specific mAb) were generously provided by Dr Mark H. Ginsberg (Scripps Research Institute).26,27 PMI-1, AP5, anti-LIBS1, and anti-LIBS6 recognize ligand-induced conformational changes on IIb3, termed ligand-induced binding sites (LIBS).21,26,27 TP80 (IIb-specific mAb) and MOPC21 (mouse myeloma immunoglobulin G1 [IgG1]) were purchased from Nichirei (Tokyo, Japan) and Sigma Chemical (St Louis, MO), respectively. FK633 (IIb3-specific peptidomimetic antagonist) was generously provided by Dr Jiro Seki (Fujisawa Pharmaceutical, Osaka Japan),28 and cyclo(RGDfV) [cyclo(-Arg-D-Gly D-Asp-D-Phe-L-Val-D-)] peptide (v3-specific antagonist) was a generous gift from Merck KGaA (Darmstadt, Germany).29 Fibrinogen was purchased from Calbiochem-Novabiochem (San Diego, CA). MOPC21, AP1, AP2, TP80, AP3, PAC-1, and fibrinogen were labeled with fluorescein isothiocyanate (FITC), as previously described.28 Flow cytometry Flow cytometric analysis using various mAbs was performed as previously described.30 FITC-labeled mAbs were used to quantify the expression levels of IIb3 on 293 cells and on platelets. Analysis of platelet messenger RNA Total cellular RNA of platelets was isolated from 30 mL whole blood, and IIb or 3 messenger RNA (mRNA) was specifically amplified by reverse transcription-polymerase chain reaction (RT-PCR), as previously described.30 Primers for the amplification of IIb or 3 mRNA and conditions for RT-PCR were described elsewhere.30,31 Nucleotide sequences of PCR products were determined by using Taq DyeDeoxy Terminator Cycle Sequencing kit and an ABI373A DNA sequencer (Applied Biosystems, Foster City, CA). Allele-specific restriction enzyme analysis Amplification of the region around exon 4 of the IIb gene was performed by using primers IIbE3, 5'-GTCGGTCGTCAGCTGGAGC-3' (sense, nucleotide [nt] 3940-3958 in the IIb gene), and IIbE4, 5'-CAGGTCGTAGCTGGCGCTTAC-3' (antisense, nt 4192-4172) and by using 250 ng DNA as a template. First-round PCR products were reamplified using primers IIbg3947S, 5'-GTCAGCTGGAGCGACGTCATTGTG-3' (sense, nt 3947-3970) and IIbE4 and then were digested with restriction enzyme ScaI. Resultant fragments were electrophoresed in a 1.5% agarose gel. Construction of IIb expression vectors and cell transfection Wild-type IIb and 3 complementary DNAs (cDNAs) cloned into a mammalian expression vector pcDNA3 (Invitrogen, San Diego, CA) were generously provided by Dr Peter J. Newman. To construct the expression vector containing the 521C (His143) form of IIb, PCR-based cartridge mutagenesis was performed as previously described.16 Platelet IIb cDNA from Osaka-12 was amplified by RT-PCR using primers IIb1 and IIb3AS, 5'-CCCACGATCAGGTCTGGGTATCCG-3' (antisense, nt 1407-1384). Second-round amplification was then performed using 1 µL first-round PCR products as a template with nested primers IIb187S, 5'-GAGAGTGGCCATCGTGGTGG-3' (sense, nt 187-206), and IIb3AS2C, 5'-CCGTTGTCATCGATGTCTACGGC-3' (antisense, nt 1364-1386). To construct the expression vector for Tyr143Ala IIb substitution, we carried out the overlapping extension PCR as previously described.16 Mismatched sense primer IIb143Ala-S, 5'-GAGCGGCCGCCGCGCCGAGGCCTCCCCCTG-3' (sense, nt 502-531, 2 bp mismatched), and antisense primer IIb143Ala-AS, 5'-CAGGGGGAGGCCTCGGCGCGGCGGCCGCTC-3' (antisense, nt 531-502, 2 bp mismatched) were synthesized. PCR was performed by using IIb cDNA as a template and primers IIb1 and IIb143Ala-AS, or primers IIb143Ala-S and IIb3AS. The 2 individually amplified PCR products were mixed and used as a template of PCR using primers IIb1 and IIb3AS. Amplified fragments were digested with restriction enzyme SacII and ClaI, and the resultant fragments were extracted using GeneClean II kit (Bio 101, La Jolla, CA). Fragments were introduced into the pcDNA3 that had been digested with SacII and ClaI. Inserted fragments were characterized by sequence analysis to verify the absence of any other substitutions and the proper insertion of the PCR cartridge into the vector. Expression vectors containing the KO mutant (2-amino acid [Arg-Thr] insertion between residues 160 and 161) IIb cDNA and Asp163AlaIIb cDNA were constructed as previously described.16 The wild-type or mutant IIb construct was cotransfected into 293 cells with the wild-type 3 construct by the calcium phosphate method, as previously described.16 The 293 cells transiently expressing mutant IIb3 were obtained and analyzed 2 days after transfection. In selected experiments, CD36 expression vector was cotransfected with IIb and 3 constructs to monitor transfection efficiency.32 In addition, stable transfectants expressing wild-type, Tyr143HisIIb3, or KO mutant IIb3 were selected for G418 resistance (Gibco BRL, Grand Island, NY) and were cultured in Dulbecco modified Eagle medium (DMEM) with 10% heat-inactivated fetal calf serum (Life Technologies, Gaithersburg, MD). Adhesion assays Adhesion assays were performed as described by Faull et al.33 Wells from 96-well microtiter plates were coated with up to 0.5 µg fibrinogen per well in 100 µL phosphate-buffered saline (PBS) and were incubated at 4°C overnight. After washing with PBS, wells were blocked with PBS containing 1% bovine serum albumin (BSA) (Sigma) for 90 minutes at 22°C. To determine background adhesion, control wells were coated with 1% BSA. Then 293 cells transiently expressing wild-type or mutant IIb3 were washed twice with PBS and resuspended in DMEM containing 0.1% BSA at a concentration of 1 × 106 cells/mL, and 100 µL aliquots cell suspension were added to wells in triplicate. The plate was incubated in a humidified 37°C incubator for 60 minutes. After washing with PBS 3 times, the adherent cells were quantified by measuring endogenous cellular acid phosphate activity in an enzyme-linked immunosorbent assay.34 For morphologic analysis, 293 cells stably expressing wild-type or mutant IIb3 were added on fibrinogen-coated glass coverslips for 60 minutes at 37°C. After they were washed with PBS, adherent cells were fixed in 3.7% formaldehyde for 10 minutes, permeabilized in 0.5% Triton X-100 in PBS for 5 minutes at room temperature, and washed twice with PBS. Cells on coverslips were stained with rhodamine-phalloidin (Sigma) and were analyzed under a fluorescence microscope (Olympus, Tokyo, Japan).35 Tyrosine phosphorylation of pp125FAK Adherent cells on fibrinogen were lysed in Triton X-100 buffer (1% Triton X-100, 25 mM Tris-HCl, 100 mM NaCl, pH 7.4, 0.1 mg/mL leupeptin, 4 µg/mL pepstatin A, 1 mM phenylmethylsulfonyl fluoride, and 10 mM benzamide) containing sodium vanadate and were scraped into microcentrifuge tubes. Lysates were incubated on ice for 30 minutes, and clarified supernatants were processed for immunoprecipitation. Focal adhesion kinase (pp125FAK) was immunoprecipitated with 1 µg rabbit polyclonal antibody specific for FAK (Santa Cruz Biotechnology, CA), and protein-G Sepharose (Pharmacia, Uppsala, Sweden). Precipitates were separated on 7.5% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and were transferred to a polyvinylidene difluoride (PVDF) membrane. Phosphotyrosine was detected with mAb 4G10. To monitor the loading of gel lanes, the blots were stripped (2% SDS, 62.5 mM Tris, pH 6.7, 100 mM 2-mercaptoethanol for 30 minutes at 70°C) and reprobed with anti-FAK.36 Clot retraction Clot retraction of stable cell lines was performed based on the method described by Katagiri et al.37 In brief, 293 cells stably expressing IIb3 in Tyrode/HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) buffer (2 × 106 cells/mL) were incubated with 10 mM tranexamic acid, 250 µg/mL fibrinogen, and 2 mM CaCl2 at 37°C in a siliconized glass tube. To block the effect of v3 composed of endogenous v and transfected 3, these stable transfectants were preincubated with 50 µM cyclo(RGDfV) (v3-specific antagonist) at room temperature for 30 minutes. Then 1 U thrombin was added to 1 mL cell suspension. Fibrin gels began to form immediately after the addition of thrombin, and the tubes were kept at 37°C. Clot retraction was monitored by taking photographs every 30 minutes using a digital camera. Quantification of retraction was performed by an assessment of the clot area using the NIH Image 1.67e software (Bethesda, MD). Data were expressed as follows: % clot retraction = [(area t0  area t)/area t0] × 100.     Results Top Abstract Introduction Patients, materials, and... Results Discussion References Platelets from thrombasthenic patient Osaka-12 show impaired ligand-binding function We first examined the surface expression of IIb3 on platelets from patient Osaka-12 and 3 control subjects using flow cytometry. Although FITC-AP1 (GPIb-specific mAb) bound to Osaka-12 platelets slightly higher than to control platelets (113% of control value [mean value obtained from 3 control subjects]), FITC-AP3 (3-specific mAb) and FITC-TP80 (IIb-specific mAb) revealed a significant reduction in the expression of IIb3 on Osaka-12 platelets (AP3 binding, 41% of control value; TP80 binding, 36% of control value; n = 2) (Figure 1). Compared with AP3 and TP80 binding to Osaka-12 IIb3, AP2 binding was markedly impaired (13% of control value; n = 2) probably because of a disturbance of AP2 epitope formation. Because Osaka-12 platelets expressed 36% to 41% of normal amounts of IIb3 on their surfaces, we examined their ligand-binding function. OP-G2 and PAC-1 mAbs are activation-independent and activation-dependent, ligand-mimetic mAbs specific for IIb3, respectively. Neither OP-G2 nor PAC-1 in the presence of an activating PT25-2 mAb bound to Osaka-12 platelets (Figure 1). Thus, Osaka-12 IIb3 seems to have a ligand-binding defect and a quantitative defect. View larger version (25K): [in this window] [in a new window]   Figure 1. Flow cytometric analysis of the surface expression and ligand-binding function of IIb3 Osaka-12 platelets. Washed platelets obtained from Osaka-12 or 3 control subjects were incubated with FITC-AP1 (GPIb-specific mAb), FITC-AP2 (IIb3-specific mAb), FITC-AP3 (3-specific mAb), and FITC-TP80 (IIb-specific mAb) at a concentration of 10 µg/mL for 30 minutes at room temperature. FITC-MOPC21 (mouse myeloma IgG1) was used as a negative control. For OP-G2 binding (activation-independent ligand-mimetic IIb3-specific mAb), bound antibodies were detected by FITC-conjugated goat F(ab')2 antimouse IgG. Filled and open histograms represent the binding of specific and control antibodies, respectively. For PAC-1 binding, washed platelets were preincubated with 10 µg/mL PT25-2 (activating IIb3-specific mAb) for 30 minutes, and then 10 µg/mL FITC-labeled PAC-1 was added. Closed and open histograms represent the PAC-1 binding in the absence and presence of 10 µM FK633 (IIb3-specific antagonist), respectively. Results are representative of 2 separate experiments. IIb3 LIBS expression on Osaka-12 platelets by IIb3-specific peptidomimetic antagonist FK633 PMI-1, AP5, anti-LIBS1, and anti-LIBS6 mAbs preferentially recognize LIBS on IIb3, which are exposed following occupancy of the receptor by ligands. To further investigate the ligand-binding function of Osaka-12 IIb3, we examined the effect of FK633, IIb3-specific peptidomimetic antagonist on the expression of these LIBS. Compared with Arg-Gly-Asp-Trp peptide, FK633 has approximately 100-fold higher potency for LIBS induction.28 Although we could not simply compare the binding of mAbs recognizing LIBS between Osaka-12 and control platelets because of the different expression levels of IIb3, even 10 µM FK633 showed a negligible effect on the expression of LIBS1 of Osaka-12 IIb3 (Table 1). On the other hand, Table 1 shows that Osaka-12 IIb3 aberrantly expressed LIBS recognized by PMI-1, AP5, and anti-LIBS6. These data further suggested that the ligand-binding sites of Osaka-12 IIb3 were markedly impaired.                                View this table: [in this window] [in a new window]   Table 1. Analysis of LIBS expression in Osaka-12 llb3 Nucleotide sequence and allele-specific restriction enzyme analyses for Osaka-12 IIb3 To identify the molecular genetic defect responsible for Osaka-12, all coding regions of IIb and 3 cDNA amplified by RT-PCR from Osaka-12 and control platelet mRNA were analyzed. Direct sequence of the PCR fragments showed a single T>C substitution at nt 521 in IIb cDNA that would lead to a Tyr143His substitution in exon 4 of IIb (Figure 2A). No other nucleotide substitutions were detected in the coding regions of either IIb or 3 cDNAs. The T>C substitution abolished a restriction site for ScaI, and allele-specific restriction enzyme analysis for the amplified IIb cDNA around exon 4 suggested that Osaka-12 might be homozygous for the substitution. However, allele-specific restriction enzyme analysis for the PCR fragments from genomic DNA revealed that Osaka-12 was heterozygous for the T>C substitution. The substitution was inherited from her father. The molecular genetic defect inherited from her mother remains determined. However, only the allele having 521C from her father was amplified by RT-PCR from Osaka-12 IIb mRNA, suggesting that the expression levels of maternal IIb mRNA seem to be extremely low in Osaka-12 platelets (Figure 2B-C). View larger version (22K): [in this window] [in a new window]   Figure 2. Identification of the genetic defect responsible for Osaka-12. (A) Nucleotide sequence analysis of IIb cDNA from Osaka-12. Platelet IIb mRNA was amplified by RT-PCR. Nucleotide sequence of the amplified fragments was determined by using Taq DyeDeoxy Terminator Cycle Sequencing kit and an ABI 373A DNA sequencer. (B) Allele-specific restriction enzyme analysis of IIb cDNA. The region around exon 4 of the IIb mRNA was amplified by PCR, followed by digestion with ScaI. ScaI digestion of the PCR products yielded 517-bp, 264-bp, and 316-bp fragments in the healthy allele. The T>C substitution abolished one of the restriction sites for ScaI. Resultant fragments were electrophoresed in a 1.5% agarose gel. Marker indicates 100-bp DNA ladder. (C) Allele-specific restriction enzyme analysis of IIb genomic DNA. The region around exon 4 of the IIb gene was amplified by PCR using primers IIb3945S and IIbE4, followed by digestion with ScaI. ScaI digestion of the PCR products yielded 241-bp and 75-bp fragments in the healthy allele. The T>C substitution abolished the restriction site for ScaI. Resultant fragments were electrophoresed in a 6% polyacrylamide gel. Marker indicates X174 digested with HaeIII. Effect of Tyr143His substitution in IIb on IIb3 expression and function To confirm that the 521T>C substitution leading to Tyr143His substitution in IIb is responsible for the functional defect, we constructed an expression vector that contained the wild-type or mutant His143 form of IIb and cotransfected it with wild-type 3 vector into 293 cells. Effects of a Tyr143Ala substitution in IIb on IIb3 expression and function were also examined. Because Tyr143 is located within the W3 4-1 loop of the -propeller domain, close to the KO mutation (the Arg-Thr insertion between 160 and 161 residues of IIb) and the Asp163Ala IIb mutation, we compared the expression and function of these mutant IIb3 in parallel. Transfection efficiency monitored by the cotransfected CD36 expression vector was essentially the same between wild-type and mutant IIb3 (Figure 3). Flow cytometric analysis using FITC-AP3 or FITC-TP80 mAb showed that the expression levels of Tyr143HisIIb3 were essentially the same as wild-type IIb3 (AP3 binding, 91% ± 15% of wild-type; TP80 binding, 102% ± 2% of wild-type; mean ± SD; n = 3). In sharp contrast, Tyr143HisIIb3 failed to bind OP-G2 mAb, FITC-PAC-1 mAb, or FITC-fibrinogen in the presence of PT25-2 mAb (Figure 3). We also confirmed that the binding failure of these activation-dependent ligands was not caused by the binding failure of the activating mAb, PT25-2. In addition, FITC-AP2 binding was significantly impaired compared with TP80 binding (AP2 binding; 15% ± 3% of wild-type; mean ± SD; n = 3), which is consistent with the data obtained from Osaka-12 platelets (Figures 1 and 3). Given that the expression levels of abnormal IIb mRNA derived from her mother were so low that they were not detected in Osaka-12 platelets by RT-PCR, this maternal abnormality would reduce the expression of IIb3 by 50%. As expected, the expression level of Tyr143HisIIb3 on 293 cells was roughly twice as much as that on Osaka-12 platelets. These data indicate that the Tyr143His mutation is responsible for the variant GT phenotype in Osaka-12. The Tyr143Ala substitution induced essentially the same effects on IIb3 as the Tyr143His mutation (AP3 binding, 117% ± 14% of wild-type; TP80 binding, 113% ± 8% of wild-type; AP2 binding, 14% ± 1% of wild-type; mean ± SD; n = 3). Because Tyr143Ala abolished the ligand binding, it is likely that the presence of Tyr143 is critical for IIb3 function. Interestingly, the phenotype of Asp163AlaIIb3 appears the same as that of Tyr143HisIIb3. The phenotype of KO IIb3 was essentially the same as wild-type IIb3 except for the failure in OP-G2 and PAC-1 binding, as previously described.15 Thus, none of these mutant IIb3 showed a binding ability for soluble ligands (Figure 3). View larger version (47K): [in this window] [in a new window]   Figure 3. Effects of Tyr143HisIIb, Tyr143AlaIIb, Asp163AlaIIb, or KO mutant on the expression and ligand-binding function of IIb3 on 293 cells. Wild-type IIb or each mutant IIb cDNA was transiently cotransfected with wild-type 3 cDNA into 293 cells. CD36 expression vector was cotransfected with IIb and 3 constructs, and CD36 expression was measured by FITC-anti-CD36 mAb to monitor transfection efficiency. The binding of FITC-AP2, FITC-AP3, FITC-TP80, OP-G2, and PT25-2 was analyzed by flow cytometry 2 days after transfection (filled histograms). FITC-MOPC21 was used as a negative control (open histograms). For OP-G2 or PT25-2 binding, bound antibodies were detected by FITC-conjugated goat F(ab')2 antimouse IgG. For PAC-1 or fibrinogen binding, washed platelets were preincubated with 10 µg/mL PT25-2 (activating IIb3-specific mAb) for 30 minutes, and then 10 µg/mL FITC-labeled PAC-1 or 150 µg/mL FITC-labeled fibrinogen was added. Closed and open histograms represent PAC-1 or fibrinogen binding in the absence and presence of 10 µM FK633 (IIb3-specific antagonist), respectively. Results are representative of 3 separate experiments. Adhesion of the cells to immobilized fibrinogen To further examine the functions of mutant IIb3, we performed cell adhesion assay to immobilized fibrinogen. In contrast to parent cells, 293 cells transiently transfected with wild-type IIb3 or mutant IIb3 became adhesive to the immobilized fibrinogen. Given that endogenous v in 293 cells can form v3 with the exogenous 3 and may contribute to the adhesion, we preincubated the transfected cells with 50 µM cyclo(RGDfV) to block the v3 function. Under these conditions, 293 cells transfected with wild-type 3 alone (data not shown), KO variant IIb3, or Asp163AlaIIb3 failed to adhere to immobilized fibrinogen (Figure 4). However, 293 cells expressing Tyr143HisIIb3 showed a significant adhesion to immobilized fibrinogen, and the amounts of adherent cells were approximately 50% compared with 293 cells expressing wild-type IIb3. The adhesion of 293 cells expressing Tyr143HisIIb3 in the presence of 50 µM cyclo(RGDfV) was mediated solely by IIb3 because 10 µM FK633 completely blocked the cell adhesion (data not shown). View larger version (25K): [in this window] [in a new window]   Figure 4. Adhesion of IIb3 mutants to immobilized fibrinogen. Wild-type IIb or each mutant IIb cDNA was transiently cotransfected with wild-type 3 cDNA into 293 cells. Two days after transfection, wild-type or mutant IIb3-transfected cells (1 × 106 cells/well) were incubated with immobilized fibrinogen (FBG) in the presence of 50 µM cyclo(RGDfV) (v3-specific antagonist) at 37°C. After washing with PBS, adherent cells were quantified with a colorimetric reaction using endogenous cellular acid phosphatase activity. Data represent the mean ± SD of triplicate measures of optical density at 415 nm. Statistical analysis (2-tailed P values for paired sample) was performed between 293 cells expressing wild-type IIb3 and those expressing each mutant IIb3 (**P < .01; *P < .05). We then obtained the cells stably expressing wild-type, Tyr143His, and KO mutant IIb3 with G418 selection for morphologic analysis of the adherent cells. Adhesive functions of 293 cells stably expressing Tyr143His IIb3 or KO IIb3 were the same as those transiently expressing these mutant IIb3 (data not shown). Although 293 cells stably expressing wild-type IIb3 adhered firmly and showed complete spreading, 293 cells stably expressing KO mutant IIb3 failed to adhere. In contrast to KO IIb3, 293 cells stably expressing Tyr143HisIIb3 moderately impaired cell spreading and cell adhesion (Figure 5). Because FAK, a 125-kDa cytoplasmic tyrosine kinase, is a component of focal adhesions and is a well-established component of integrin signaling pathways,38 the tyrosine phosphorylation of pp125FAK in these adherent 293 cells stably expressing wild-type IIb3 or Tyr143HisIIb3 was examined. As shown in Figure 6, pp125FAK phosphorylation was observed but impaired in cells expressing Tyr143HisIIb3 compared with wild-type IIb3 (Figure 6). View larger version (21K): [in this window] [in a new window]   Figure 5. Morphologic analysis of the adherent cells stably expressing mutant IIb3 to immobilized fibrinogen. 293 cells stably expressing (A) wild-type IIb3, (B) Tyr143HisIIb3, or (C) KO mutant IIb3 were incubated with immobilized fibrinogen (5 µg/mL) at 37°C for 60 minutes in the presence of 50 µM cyclo(RGDfV) (v3-specific antagonist). After gentle washing with PBS, adherent cells were fixed in 3.7% formaldehyde and permeabilized in 0.5% Triton X-100. Cells were stained with rhodamine-phalloidin and analyzed under a fluorescence microscope. Original magnification, × 100. View larger version (32K): [in this window] [in a new window]   Figure 6. Tyrosine phosphorylation of pp125FAK. Nonadherent or adherent 293 cells stably expressing wild-type IIb3 (lanes 1 and 3) or Tyr143HisIIb3 (lanes 2 and 4) on fibrinogen were lysed in Triton X-100 buffer containing sodium vanadate. pp125FAK was immunoprecipitated with 1 µg rabbit polyclonal antibody specific for FAK. Precipitates were separated on 7.5% SDS-PAGE and transferred to a PVDF membrane. Phosphotyrosine was detected with monoclonal antibody 4G10. To monitor the loading of gel lanes, the blots were reprobed with anti-FAK. Clot retraction We examined the capacity of Tyr143HisIIb3 and KO mutant IIb3 to mediate fibrin clot retraction by using stable transfectants. This process reflects the capacity of IIb3 to transduce mechanical force between the intracellular cytoskeleton and the extracellular fibrin strands. Parent 293 cells failed to retract fibrin clots. Given that v3 is involved in mediating fibrin clot retraction,37 we preincubated all stable transfectants with 50 µM cyclo(RGDfV) to block the v3 function. Under these conditions, 293 cells stably transfected with wild-type 3 alone failed to mediate fibrin clot retraction, whereas those with wild-type IIb and 3 showed a marked retraction. Although 293 cells stably expressing Tyr143HisIIb3 showed some ability for fibrin clot retraction (Figure 7), 293 cells stably expressing KO mutant IIb3 failed to mediate fibrin clot retraction. View larger version (20K): [in this window] [in a new window]   Figure 7. Fibrin clot retraction mediated by the cells stably expressing mutant IIb3. 293 cells (control), 293 cells stably expressing wild-type 3 alone, wild-type IIb3, Tyr143His