Evaluation of Platelet Membrane Glycoproteins in Coronary Artery Disease

Abstract

Platelets play a fundamental role in atherogenesis and development of ischemic complications. [1][2][3] Under physiological conditions, platelets do not interact with the vessel wall. Injury of vascular intima disrupts the antithrombotic properties of endothelium and exposes the blood to adhesive molecules of the subendothelium. Platelet adhesion to the damaged vessel wall is the first step in hemostasis and thrombosis. 4 Platelet adhesion is followed by spreading and activation, resulting in release of granule components and aggregate formation. 5,6 On initial contact, platelet glycoprotein (GP) Ib/V/IX complex binds to von Willebrand factor associated with collagen on the subendothelial surface (Figure 1) 4,5 and thus arrests the platelet on the vessel surface. The collagen receptor ␣ 2 ␤ 1 is an important secondary receptor for platelet adhesion. ␣ 2 ␤ 1 -Collagen interaction leads to platelet activation and is critical for the spreading process involving the fibrinogen receptor GP IIb/IIIa to ensure close contact of the spread platelet with the surface. 5 Other adhesion receptors, including the fibronectin receptor ␣ 5 ␤ 1 and the laminin receptor ␣ 6 ␤ 1 , support and strengthen secondary adhesion ( Figure 1). The fibrinogen receptor GP IIb/IIIa is particularly important in platelet-platelet coadhesion, termed aggregation. This requires conformational changes in GP IIb/IIIa that allow binding of soluble fibrinogen to the platelet membrane ( Figure 2). Thus, fibrinogen bridging allows formation of platelet aggregates (Figure 2). 6 -9 Platelet adhesion and aggregation induce intracellular signaling, which mediates several responses, such as formation and secretion of thromboxane A2 (TXA 2 ), serotonin, and ADP 6 (Figure 1). These substances reinforce platelet activation, vasoconstriction, and slowing of blood flow and therefore increase platelet-platelet and platelet-vessel wall interaction. 3,6 Platelet Membrane ReceptorsThe most abundant platelet membrane glycoprotein is the ␤ 3 -integrin GP IIb/IIIa (60 000 to 100 000 per platelet and 1% to 2% of the total platelet protein), the inducible platelet fibrinogen receptor. 7 Eighty percent of GP IIb/IIIa is randomly distributed and expressed on the platelet surface in its resting state, and the remaining 20% is located within the surface connecting system (SCS) and in ␣-granule membranes 5,10 (Figure 3). GP IIb/IIIa stored in this internal pool becomes surface expressed as functional receptor on platelet activation. 11 Congenital deficiencies of GP IIb/IIIa in Glanzmann thrombasthenia lead to defective platelet aggregation and enhanced bleeding. 5 As is the case with other integrins, GP IIb/IIIa is a heterodimer consisting of an ␣-subunit (GP IIb) and a ␤-subunit (GP IIIa) 9 (Figure 4). Although the expression of GP IIb/IIIa (␣ IIb ␤ 3 ) is limited to megakaryocytes and platelets, the other ␤ 3 -integrin present on platelets, the vitronectin receptor ␣ v ␤ 3 , is more widely distributed and is also found on endothelial and smooth muscle cells. 12 ␣ v ␤ 3 shar...

Clopidogrel for Coronary Stenting

“…Moreover, we assessed the expression of an established marker of platelet activation (P-selectin) in response to a maximal agonist concentration and studied the response of a sensitive platelet activationdependent marker (PAC-1 binding) in nonstimulated blood. 5,9 The present study suggests that the maximum inhibitory response to a 300-mg loading dose followed by 75 mg/d occurs within 24 hours. These findings are consistent with Figure 5.…”

“…The PAC-1 antibody (Becton Dickinson) binds only to the active ␣ IIb ␤ 3 receptor, and therefore the total amount of ␣ IIb ␤ 3 is not determined. 9 PAC-1 was expressed as log mean fluorescence intensity. P-selectin (Pharmingen) was measured after stimulation with 200 mol/L ADP and is expressed as percent positivity (ie, the percentage of platelets positive for the antibody) as described previously.…”

Retroviral Infection and Selection of Culture-Derived Platelets Allows Study of the Effect of Transgenes on Platelet Physiology Ex Vivo and on Thrombus Formation In Vivo

“…12 Serotonin release measurements were performed as described. 1 ELISA assays for serotonin (DLD Diagnostics) were performed according to manufacturer instructions.…”

Loading With 600 mg Clopidogrel in Patients With Coronary Artery Disease With and Without Chronic Clopidogrel Therapy

“…10 The following monoclonal antibodies were used: anti-CD61 PE (anti-GP IIb/IIIa labeled with phycoerythrine; Serotec) and anti-CD62 FITC (anti-P-selectin labeled with fluorescein isothiocyanate; Beckman Coulter). In brief, citrated blood …”