Expression Of GPIbα Research Articles

Naturally Occurring Mutations in Glycoprotein Ibα That Result in Defective Ligand Binding and Synthesis of a Truncated Protein

The platelet GPIb-V-IX complex is the receptor for the initial binding of von Willebrand factor (vWF) mediating platelet adhesion. The complex is composed of four membrane-spanning glycoproteins (GP): GPIbalpha, GPIbbeta, GPIX, and GPV. Bernard-Soulier syndrome results from a qualitative or quantitative defect in one or more components of the platelet membrane GPIb-V-IX complex. We describe the molecular basis of a novel Bernard-Soulier syndrome variant in two siblings in whom GPIbalpha was not detected on the platelet surface but that was present in a soluble form in plasma. DNA sequence analysis showed that the affected individuals were compound heterozygotes for two mutations. One, inherited from a maternal allele, a T777 --> C point mutation in GPIbalpha converting Cys65 --> Arg within the second leucine rich repeat, the other, a single nucleotide substitution (G2078 --> A) for the tryptophan codon (TGG) causing a nonsense codon (TGA) at residue 498 within the transmembrane region of GPIbalpha, inherited from a mutant paternal allele. The Bernard-Soulier phenotype was observed in siblings who were compound heterozygotes for these two mutations. Although GPIbalpha was not detected on the surface of the patient's platelets, soluble GPIbalpha could be immunoprecipitated from plasma. When plasmids encoding GPIbalpha containing the Cys65 --> Arg mutation were transiently transfected into Chinese hamster ovary (CHO) cells stably expressing the GPbeta-IX complex (CHObetaIX), the expression of GPIbalpha was similar to the wild-type (WT) GPIbalpha, but did not bind vWF. When plasmids encoding GPIbalpha containing the Trp498 --> stop were transiently transfected into CHObetaIX, the surface expression of GPIbalpha was barely detectable compared with the WT GPIbalpha. Thus, this newly described compound heterozygous defect produces Bernard-Soulier syndrome by a combination of synthesis of a nonfunctional protein and of a truncated protein that fails to insert into the platelet membrane and is found circulating in plasma.

A GPIbα-Related Protein Is Expressed by Fresh Human Breast Carcinoma Tissue and Is Regulated by a PKC-Sensitive Mechanism

Fresh frozen breast carcinoma tissues were examined for the presence of GPIbα by immunohistochemistry. GPIbα was detected in six of seven primary invasive intraductal breast carcinoma tissues whereas staining was negative in seven of seven nonmalignant breast specimens. When biotin-labeled, triton-lysed, phorbol-12-myristate 13-acetate (PMA)-incubated breast carcinoma MCF-7 cells were immunoprecipitated with a MoAb directed against the platelet GPIb/IX complex, expression of GPIb was significantly enhanced in comparison to control preparations. Furthermore, incubation of MCF-7 cells for 84 h with 16 nmol/L PMA, but not with its biologically inactive derivative MePMA, induced a three- to fourfold increase in the surface expression of both GPIbα and GPIb/IX by flow cytometry. This PMA-enhanced GPIbα expression was almost completely abrogated when MCF-7 cells were first preincubated with the specific protein kinase C (PKC) inhibitor, H-7, prior to PMA treatment. Finally, PMA-incubated MCF-7 cells demonstrated a 63% ( N= 6; P< 0.001) increase in tumor-induced platelet agglutination when added to platelets in comparison to control tumor cells. This enhancement could be abrogated by H-7. These findings confirm the expression of a protein with homology to platelet GPIbα expressed by fresh human breast carcinoma tissues, demonstrate that PMA enhances GPIb membrane expression by MCF-7 cells, and suggest that PKC plays a role in this process.

Relative Importance of the Glycoprotein Ib-Binding Domain and the RGD Sequence of von Willebrand Factor for Its Interaction With Endothelial Cells

AbstractEndothelial cell adhesion to von Willebrand Factor is mainly mediated through an interaction between the αvβ3 integrin and the RGD sequence of von Willebrand factor (vWF ). To define the potential involvement of glycoprotein Ibα (GPIbα) as an endothelial vWF receptor, we compared cell adhesion to three recombinant vWF, the wild-type (WT-rvWF ) and two mutants, RGGS-rvWF (D1746G), defective for binding to platelet αIIbβ3, and ΔA1-rvWF with a deletion between amino-acids 478 and 716, which does not bind to platelet GPIbα. Adhesion of human umbilical vein endothelial cells to purified vWF recombinants was measured by automatized cell counting using an image analyzer. Whereas cell adhesion to ΔA1-rvWF was unchanged compared with WT-rvWF, reaching a plateau of 40% total cells at a concentration of 2.5 μg/mL rvWF, adhesion to RGGS-rvWF was only 10% of total cells. Cell stimulation by tumor necrosis factor-α (TNFα), reported to upregulate the expression of the putative endothelial GPIbα, did not modify adhesion to these rvWF. Monoclonal antibodies to vWF or GPIbα, blocking vWF interaction with platelet GPIbα, were unable to inhibit endothelial cell adhesion to rvWF. In contrast, antibody 9 to vWF, blocking the αvβ3-dependent endothelial cell adhesion to plasma vWF, inhibited adhesion to WT-rvWF as efficiently as to ΔA1-rvWF (50% inhibition at a concentration of 11 and 15 μg/mL, respectively). In agreement with the fact that endothelial cell adhesion to vWF appeared independent of the GPIbα-binding domain, we were unable to detect endothelial surface expression of GPIbα by flow cytometry or in cell lysates by immunoprecipitation followed by immunoblotting. Moreover, expression of GPIbα mRNA was undetectable in endothelial cells, even after stimulation by TNFα. These studies indicate that GPIbα is not expressed in human cultured endothelial cells and is not involved in adhesion to vWF-containing surfaces. Thus, in static conditions, cultured endothelial cells adhere to vWF through an αvβ3-dependent, GPIbα-independent mechanism.

Relative Importance of the Glycoprotein Ib-Binding Domain and the RGD Sequence of von Willebrand Factor for Its Interaction With Endothelial Cells

Endothelial cell adhesion to von Willebrand Factor is mainly mediated through an interaction between the αvβ3 integrin and the RGD sequence of von Willebrand factor (vWF ). To define the potential involvement of glycoprotein Ibα (GPIbα) as an endothelial vWF receptor, we compared cell adhesion to three recombinant vWF, the wild-type (WT-rvWF ) and two mutants, RGGS-rvWF (D1746G), defective for binding to platelet αIIbβ3, and ΔA1-rvWF with a deletion between amino-acids 478 and 716, which does not bind to platelet GPIbα. Adhesion of human umbilical vein endothelial cells to purified vWF recombinants was measured by automatized cell counting using an image analyzer. Whereas cell adhesion to ΔA1-rvWF was unchanged compared with WT-rvWF, reaching a plateau of 40% total cells at a concentration of 2.5 μg/mL rvWF, adhesion to RGGS-rvWF was only 10% of total cells. Cell stimulation by tumor necrosis factor-α (TNFα), reported to upregulate the expression of the putative endothelial GPIbα, did not modify adhesion to these rvWF. Monoclonal antibodies to vWF or GPIbα, blocking vWF interaction with platelet GPIbα, were unable to inhibit endothelial cell adhesion to rvWF. In contrast, antibody 9 to vWF, blocking the αvβ3-dependent endothelial cell adhesion to plasma vWF, inhibited adhesion to WT-rvWF as efficiently as to ΔA1-rvWF (50% inhibition at a concentration of 11 and 15 μg/mL, respectively). In agreement with the fact that endothelial cell adhesion to vWF appeared independent of the GPIbα-binding domain, we were unable to detect endothelial surface expression of GPIbα by flow cytometry or in cell lysates by immunoprecipitation followed by immunoblotting. Moreover, expression of GPIbα mRNA was undetectable in endothelial cells, even after stimulation by TNFα. These studies indicate that GPIbα is not expressed in human cultured endothelial cells and is not involved in adhesion to vWF-containing surfaces. Thus, in static conditions, cultured endothelial cells adhere to vWF through an αvβ3-dependent, GPIbα-independent mechanism.

Heterogenous Expression of Glycoprotein lb, IX and V in Platelets from Two Patients with Bernard-Soulier Syndrome Caused by Different Genetic Abnormalities

Bernard-Soulier syndrome (BSS) is a rare inherited bleeding disorder, which is caused by deficiency or decrease of the platelet GPIb/IX/V complex. Analysis of two patients with BSS by flow cytometry of the blood revealed different expression patterns of the components of the GPIb/IX/V complex. In case 1, GPIX was completely absent but residual amounts of GPIb alpha and GPV were detectable; in case 2, GPIb alpha was completely absent. We amplified the coding regions of GPIb alpha, GPIb beta, GPV, and GPIX from the patients' genomic DNA with the polymerase chain reaction (PCR) and sequenced the PCR products. in case 1, we identified a point mutation in the GPIX coding region that changes the codon for tryptophan-126 (TGG) to a nonsense codon (TGA). In case 2, we found a deletion of nucleotide within seven adenine repeats at the position of 1932 to 1938 in the coding region of GPIb alpha, which causes a frame shift that results in 58 altered amino acids and a premature stop codon. These genetic changes alter the transmembrane domain of GPIX or GPIb alpha and, therefore, would prevent proper insertion of the proteins in the plasma membrane. Thus, abnormality of a single component protein (GPIX or GPIb alpha) alters the assembly of the GPIb/IX/V complex and causes heterogeneous surface expression of GPIb alpha, GPV and GPIX.