Ca(2+)-dependent structural transitions of the platelet glycoprotein IIb-IIIa complex. Preparation of stable glycoprotein IIb and IIIa monomers

The regulation of integrin function by divalent cations

Integrins are a family of α/β heterodimeric adhesion metalloprotein receptors and their functions are highly dependent on and regulated by different divalent cations. Recently advanced studies have revolutionized our perception of integrin metal ion-binding sites and their specific functions. Ligand binding to integrins is bridged by a divalent cation bound at the MIDAS motif on top of either α I domain in I domain-containing integrins or β I domain in α I domain-less integrins. The MIDAS motif in β I domain is flanked by ADMIDAS and SyMBS, the other two crucial metal ion binding sites playing pivotal roles in the regulation of integrin affinity and bidirectional signaling across the plasma membrane. The β-propeller domain of α subunit contains three or four β-hairpin loop-like Ca(2+)-binding motifs that have essential roles in integrin biogenesis. The function of another Ca(2+)-binding motif located at the genu of α subunit remains elusive. Here, we provide an overview of the integrin metal ion-binding sites and discuss their roles in the regulation of integrin functions.

Acute thrombocytopenia after treatment with tirofiban or eptifibatide is associated with antibodies specific for ligand-occupied GPIIb/IIIa

Acute thrombocytopenia is a recognized complication of treatment with GPIIb/IIIa inhibitors whose cause is not yet known. We studied 9 patients who developed severe thrombocytopenia (platelets less than 25 × 109/L) within several hours of treatment with the GPIIb/IIIa inhibitors tirofiban (4 patients) and eptifibatide (5 patients). In each patient, acute-phase serum contained a high titer (range, 1:80-1:20 000) IgG antibody that reacted with the glycoprotein IIb/IIIa complex only in the presence of the drug used in treatment. Four patients had been previously treated with the same drug, but 5 had no known prior exposure. Pretreatment serum samples from 2 of the latter patients contained drug-dependent antibodies similar to those identified after treatment. No tirofiban- or eptifibatide-dependent antibodies were found in any of 100 randomly selected healthy blood donors, and only 2 of 23 patients receiving tirofiban or eptifibatide who did not experience significant thrombocytopenia had extremely weak (titer, 1:2) tirofiban-dependent antibodies. In preliminary studies, evidence was obtained that the 9 antibodies recognize multiple target epitopes on GPIIb/IIIa complexed with the inhibitor to which the patient was sensitive, indicating that they cannot all be specific for the drug-binding site. The findings indicate that acute thrombocytopenia after the administration of tirofiban or eptifibatide can be caused by drug-dependent antibodies that are “naturally occurring” or are induced by prior exposure to drug. These antibodies may be human analogs of mouse monoclonal antibodies that recognize ligand-induced binding sites (LIBS) induced in the GPIIb/IIIa heterodimer when it reacts with a ligand-mimetic drug.

Acute thrombocytopenia after treatment with tirofiban or eptifibatide is associated with antibodies specific for ligand-occupied GPIIb/IIIa

Acute thrombocytopenia is a recognized complication of treatment with GPIIb/IIIa inhibitors whose cause is not yet known. We studied 9 patients who developed severe thrombocytopenia (platelets less than 25 × 109/L) within several hours of treatment with the GPIIb/IIIa inhibitors tirofiban (4 patients) and eptifibatide (5 patients). In each patient, acute-phase serum contained a high titer (range, 1:80-1:20 000) IgG antibody that reacted with the glycoprotein IIb/IIIa complex only in the presence of the drug used in treatment. Four patients had been previously treated with the same drug, but 5 had no known prior exposure. Pretreatment serum samples from 2 of the latter patients contained drug-dependent antibodies similar to those identified after treatment. No tirofiban- or eptifibatide-dependent antibodies were found in any of 100 randomly selected healthy blood donors, and only 2 of 23 patients receiving tirofiban or eptifibatide who did not experience significant thrombocytopenia had extremely weak (titer, 1:2) tirofiban-dependent antibodies. In preliminary studies, evidence was obtained that the 9 antibodies recognize multiple target epitopes on GPIIb/IIIa complexed with the inhibitor to which the patient was sensitive, indicating that they cannot all be specific for the drug-binding site. The findings indicate that acute thrombocytopenia after the administration of tirofiban or eptifibatide can be caused by drug-dependent antibodies that are “naturally occurring” or are induced by prior exposure to drug. These antibodies may be human analogs of mouse monoclonal antibodies that recognize ligand-induced binding sites (LIBS) induced in the GPIIb/IIIa heterodimer when it reacts with a ligand-mimetic drug.

Platelet proteins as autoantibody targets in idiopathic thrombocytopenic purpura

Idiopathic thrombocytopenic purpura (ITP), caused by autoantibodies directed against certain platelet antigens, is the most common entity of the immune thrombocytopenias. ITP is an acquired disorder and can affect both children and adults. However, the clinical syndromes of ITP are distinct between children and adults. Childhood (acute) ITP characteristically is acute in onset, occurs within 1-2 weeks of an infection, usually of viral origin, resolves spontaneously within 6 months. Adult (chronic) ITP has an insidious onset and rarely resolves spontaneously. Over the last decade considerable new information has accumulated as to the pathophysiological mechanisms of immune thrombocytopenias. In addition, most of the knowledge on this disorder has been obtained from studies of adult patients with chronic ITP. The present work gives an updated overview of the platelet autoantigens and the molecular immunological reactions in ITP.

Inherited mutations within the calcium-binding sites of the integrin alpha IIb subunit (platelet glycoprotein IIb). Effects of the amino acid side chain and the amino acid position on cation binding

To examine the effects of naturally occurring inherited mutations on the ability of the integrin alpha-subunit, also termed glycoprotein IIb (GPIIb), to bind metal ions, we prepared small synthetic peptides that encompassed individual cation-binding domains, and recombinant GPIIb poly peptides that encompassed all four Ca(2+)-binding domains, and examined their interactions with divalent cations by means of Tb3+-luminescence spectroscopy. Replacement of the highly conserved Gly418 residue, located within the flanking region of the fourth Ca(2+)-binding domain of GPIIb, with a negatively charged Asp residue resulted in marked reduction in the ability to bind divalent cations. A variant form of GPIIb with a deletion of two amino acids at the -1 and X positions of the fourth Ca(2+)-binding domain of GPIIb also failed to bind metal ions in a normal manner. In contrast, a Glanzmann mutation at the -1 position of the first Ca(2+)-binding domain of GPIIb had no effect on divalent-cation-binding ability with either synthetic peptides or recombinant GPIIb polypeptides. These data support the hypothesis that the highly conserved Gly normally found 7-8 residues N-terminal to integrin metal-binding domains plays a critical role in the maintenance of the conformation or orientation of surrounding EF-hand structures so that they can effectively interact with and bind divalent cations. Furthermore, inherited mutations at or near the divalent-cation-binding domains of GPIIb do not necessarily exert their biochemical effects by disruption of cation binding, but can interfere with integrin biogenesis in a Ca(2+)-independent manner.

Characterization of cation-binding sequences in the platelet integrin GPIIb-IIIa (alpha IIb beta 3) by terbium luminescence

The binding of cations to purified GPIIb-IIIa (alpha IIb beta 3) and synthetic peptides corresponding to the potential cation-binding sites within this integrin has been assessed by terbium luminescence spectroscopy. Tb3+ supported fibrinogen binding to purified GPIIb-IIIa, at lower concentrations than Ca2+, consistent with its higher affinity for cation-binding motifs. Titration analyses indicated the presence of five Tb(3+)-binding sites of relatively high affinity in the receptor. These sites also could be filled by divalent cations. Six sequences within GPIIb-IIIa have the appropriate spacing of five of the usual six coordination sites for cations in functional Ca(2+)-binding EF-hand motifs. Peptides containing Tyr and/or Trp at selected positions as fluorescence energy donors were synthesized, and their Tb(3+)-binding capacity was assessed. The four potential Ca(2+)-binding sequences in the GPIIb subunit, GPIIb 242-255, 296-309, 364-377, and 425-438, were functional, despite lacking the usual Glu residue at the terminal coordination position. These peptides bound Tb3+ with the same affinity as typical Ca(2+)-binding loop peptides and also bound Ca2+ and other divalent cations without preference. Of the two candidate GPIIIa sequences, 118-131 and 208-221, the former bound Tb3+ and divalent cations with an affinity similar to that of the GPIIb peptides, whereas the latter peptide was not functional. This functional difference, as well as data obtained with substituted peptides, emphasizes the importance of the first coordination position for interaction of synthetic peptide loops with cations. Together, these data identify the five cation-binding sites within intact GPIIb-IIIa.

Functional properties of alternatively spliced forms of the Drosophila PS2 integrin alpha subunit

The Drosophila alpha PS2 protein is encoded by two alternatively spliced transcripts. The respective alpha PS2 proteins differ by the presence of 25 amino acids in the alpha PS2(C) protein, not found in the alpha PS2(m8) subunit, in a region thought to be important for ligand binding. We examined the functional properties of Drosophila S2 cells transformed with genes expressing either of these proteins, in association with a beta PS subunit. Both PS2 integrins support cell spreading on vertebrate vitronectin or, to a lesser extent, on fibronectin. Interestingly, the PS2(C) form promotes spreading more efficiently on vitronectin than does the PS2(m8) form, with an opposite relative efficiency seen for fibronectin. Also, the two forms of PS2 show different requirements for divalent cations in order to mediate efficient cell spreading. These divalent cations are not required to maintain the association of alpha and beta subunits. Spreading of both cell types is similarly RGD sensitive, and both PS2 integrins appear to associate with the actin cytoskeleton. To our knowledge, this represents the first demonstration of functional differences in integrin subunits resulting from splicing variation to generate different extracellular, ligand binding domains.