Effect of mutagenesis of GPIIb amino acid 273 on the expression and conformation of the platelet integrin GPIIb-IIIa

Elevated expression of integrin alpha(IIb) beta(IIIa) in drug-naïve, first-episode schizophrenic patients

BACKGROUND

Patients with schizophrenia have an increased risk over the general public of developing cardiovascular illness. It is unknown if there are functional changes in platelet surface receptors in schizophrenia. We therefore analyzed the surface expression of glycoprotein (GP)Ib, the integrin receptor alpha(IIb)beta(IIIa), CD62 (P-selectin), and CD63, and investigated platelet function in schizophrenic patients compared with healthy volunteers.

METHODS

Nineteen drug-naive, first-episode patients with a DSM IV diagnosis of paranoid schizophrenia were compared with matched healthy controls. Flow cytometry was used to assess platelet surface expression levels of GPIb, alpha(IIb)beta(IIIa), CD62, and CD63. Adenosine diphosphate-induced platelet aggregation was assayed.

RESULTS

The schizophrenic patients had a significantly (p < .0001) increased number of 68,145 +/- 8,260.1 alpha(IIb)beta(IIIa) receptors, platelet compared with 56,235 +/- 8,079.4 receptors, platelet in healthy controls.

CONCLUSIONS

Patients with schizophrenia have increased platelet expression of alpha(IIb)beta(IIIa), which may contribute to their increased risk of cardiovascular illness compared with the general population.

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

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

Defining an Antigenic Epitope on Platelet Factor 4 Associated With Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a potentially serious complication of heparin therapy. Antibodies to platelet factor 4 (PF4)/heparin complexes have been implicated in the pathogenesis of this disorder, but the antigenic epitope(s) on the protein have not been defined. To address this issue, we studied the binding of HIT antibodies to a series of recombinant proteins containing either point mutations in PF4 or chimeras containing various domains of PF4 and the related protein, neutrophil activating peptide-2 (NAP-2). Serum samples from 50 patients with a positive 14C-serotonin release assay (14C-SRA) and a clinical diagnosis of HIT and 20 normal controls were studied. HIT antibodies reacted strongly with wild-type (WT) PF4/heparin complexes, but reacted little, if at all, with NAP-2/heparin complexes (optical density [OD]405 = 2.5 and 0.2, respectively). Alanine substitutions at three of the four lysine residues implicated in heparin binding, K62, K65, and K66, had little effect on recognition by HIT antibodies (OD405 = 2.2, 2.8, and 2.0, respectively), whereas an alanine substitution at position K61 led to reduced, but still significant binding (OD405 = 1.0). Similar studies involving chimeras between PF4 and NAP-2 localized a major antigenic site to the region between the third and fourth cysteine residues for more than half of the sera tested. This site appears to involve a series of amino acids immediately after the third cysteine residue beginning with P37. Thus our studies suggest that whereas the C-terminal lysine residues of PF4 are important for heparin binding, they do not comprise a critical antigenic site for most HIT antibodies. Rather, we propose that maintaining a region near the third cysteine residue of PF4, distal from the proposed heparin-binding domain, is required to form the epitope recognized by many HIT antibodies.

© 1998 by The American Society of Hematology.

Defining an Antigenic Epitope on Platelet Factor 4 Associated With Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a potentially serious complication of heparin therapy. Antibodies to platelet factor 4 (PF4)/heparin complexes have been implicated in the pathogenesis of this disorder, but the antigenic epitope(s) on the protein have not been defined. To address this issue, we studied the binding of HIT antibodies to a series of recombinant proteins containing either point mutations in PF4 or chimeras containing various domains of PF4 and the related protein, neutrophil activating peptide-2 (NAP-2). Serum samples from 50 patients with a positive 14C-serotonin release assay (14C-SRA) and a clinical diagnosis of HIT and 20 normal controls were studied. HIT antibodies reacted strongly with wild-type (WT) PF4/heparin complexes, but reacted little, if at all, with NAP-2/heparin complexes (optical density [OD]405 = 2.5 and 0.2, respectively). Alanine substitutions at three of the four lysine residues implicated in heparin binding, K62, K65, and K66, had little effect on recognition by HIT antibodies (OD405 = 2.2, 2.8, and 2.0, respectively), whereas an alanine substitution at position K61 led to reduced, but still significant binding (OD405 = 1.0). Similar studies involving chimeras between PF4 and NAP-2 localized a major antigenic site to the region between the third and fourth cysteine residues for more than half of the sera tested. This site appears to involve a series of amino acids immediately after the third cysteine residue beginning with P37. Thus our studies suggest that whereas the C-terminal lysine residues of PF4 are important for heparin binding, they do not comprise a critical antigenic site for most HIT antibodies. Rather, we propose that maintaining a region near the third cysteine residue of PF4, distal from the proposed heparin-binding domain, is required to form the epitope recognized by many HIT antibodies.

© 1998 by The American Society of Hematology.

A Gln747→Pro Substitution in the IIb Subunit Is Responsible for a Moderate IIbβ3Deficiency in Glanzmann Thrombasthenia

To clarify a molecular defect responsible for moderate IIbβ3 deficiency, we examined two unrelated patients, MT and MS, suffering from type II and type I Glanzmann thrombasthenia (GT), respectively. Sequence analysis of polymerase chain reaction (PCR) fragments derived from platelet mRNA showed a single A→C substitution at nucleotide (nt) 2334 leading to a Gln747→ Pro in IIb in both patients. Allele-specific restriction enzyme analysis (ASRA) of genomic DNA demonstrated that patient MT was homozygous for the Gln747→Pro substitution and patient MS was compound heterozygous for this substitution and for an RNA splice mutation at the consensus sequence of the splice acceptor site of exon 18 (AG→AA). Furthermore, ASRA showed that, among 17 unrelated Japanese GT patients, this Gln747→Pro substitution was detected in 4 patients, including MT and MS (homozygous, 2 patients; heterozygous, 2 patients). Cotransfection of Pro747IIb and β3 constructs into 293 cells resulted in moderate reduction in the amount of IIbβ3 within the transfected cells as well as on the cell surface. However, Pro747IIbβ3 bound the ligand mimetic monoclonal antibody (MoAb) PAC-1 after activation of IIbβ3 by the MoAb PT25-2, suggesting that the mutant IIbβ3 possesses the ligand-binding function. The association between the mutant proIIb and β3 was not disturbed. Surface labeling and pulse chase study showed that the Gln747→Pro substitution moderately impaired both intracellular transport of the IIbβ3 heterodimers to the Golgi apparatus and endoproteolytic cleavage of proIIb into heavy and light chains. By contrast, replacement of Gln747 with Ala by mutagenesis did not impair IIbβ3expression on the cell surface. These results suggest that the presence of Pro, rather than the absence of Gln, at amino acid residue 747 on IIb is responsible for moderate IIbβ3 deficiency.

© 1998 by The American Society of Hematology.

A Gln747→Pro Substitution in the IIb Subunit Is Responsible for a Moderate IIbβ3Deficiency in Glanzmann Thrombasthenia

To clarify a molecular defect responsible for moderate IIbβ3 deficiency, we examined two unrelated patients, MT and MS, suffering from type II and type I Glanzmann thrombasthenia (GT), respectively. Sequence analysis of polymerase chain reaction (PCR) fragments derived from platelet mRNA showed a single A→C substitution at nucleotide (nt) 2334 leading to a Gln747→ Pro in IIb in both patients. Allele-specific restriction enzyme analysis (ASRA) of genomic DNA demonstrated that patient MT was homozygous for the Gln747→Pro substitution and patient MS was compound heterozygous for this substitution and for an RNA splice mutation at the consensus sequence of the splice acceptor site of exon 18 (AG→AA). Furthermore, ASRA showed that, among 17 unrelated Japanese GT patients, this Gln747→Pro substitution was detected in 4 patients, including MT and MS (homozygous, 2 patients; heterozygous, 2 patients). Cotransfection of Pro747IIb and β3 constructs into 293 cells resulted in moderate reduction in the amount of IIbβ3 within the transfected cells as well as on the cell surface. However, Pro747IIbβ3 bound the ligand mimetic monoclonal antibody (MoAb) PAC-1 after activation of IIbβ3 by the MoAb PT25-2, suggesting that the mutant IIbβ3 possesses the ligand-binding function. The association between the mutant proIIb and β3 was not disturbed. Surface labeling and pulse chase study showed that the Gln747→Pro substitution moderately impaired both intracellular transport of the IIbβ3 heterodimers to the Golgi apparatus and endoproteolytic cleavage of proIIb into heavy and light chains. By contrast, replacement of Gln747 with Ala by mutagenesis did not impair IIbβ3expression on the cell surface. These results suggest that the presence of Pro, rather than the absence of Gln, at amino acid residue 747 on IIb is responsible for moderate IIbβ3 deficiency.

© 1998 by The American Society of Hematology.

Integrin associated proteins

Integrin cytoplasmic domains may interact directly with serveral different cytoskeletal proteins and intracellular signaling molecules. Also, integrins interact directly with other transmembrane structures, including transmembrane-4 superfamily (TM4SF) proteins. New evidence suggests that TM4SF proteins may act as linkers between extracellular integrin alpha chain domains and intracellular signaling molecules, such as phosphatidylinositol 4-kinase and protein kinase C.

Expression and secretion of defined cutinase variants by Aspergillus awamori

Several cutinase variants derived by molecular modelling and site-directed mutagenesis of a cutinase gene from Fusarium solani pisi are poorly secreted by Saccharomyces cerevisiae. The majority of these variants are successfully produced by the filamentous fungus Aspergillus awamori. However, the L51S and T179Y mutations caused reductions in the levels of extracellular production of two cutinase variants by A. awamori. Metabolic labelling studies were performed to analyze the bottleneck in enzyme production by the fungus in detail. These studies showed that because of the single L51S substitution, rapid extracellular degradation of cutinase occurred. The T179Y substitution did not result in enhanced sensitivity towards extracellular proteases. Presumably, the delay in the extracellular accumulation of this cutinase variant is caused by the enhanced hydrophobicity of the molecule. Overexpression of the A. awamori gene encoding the chaperone BiP in the cutinase-producing A. awamori strains had no significant effect on the secretion efficiency of the cutinases. A cutinase variant with the amino acid changes G28A, A85F, V184I, A185L, and L189F that was known to aggregate in the endoplasmic reticulum of S. cerevisiae, resulting in low extracellular protein levels, was successfully produced by A. awamori. An initial bottleneck in secretion occurred before or during translocation into the endoplasmic reticulum but was rapidly overcome by the fungus.

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.