Fibrinogen and glycoprotein IIb/IIIa localization during platelet adhesion. Localization to the granulomere and at sites of platelet interaction.

Platelet membrane glycoprotein IIb-IIIa plays a focal role in primary hemostasis by serving as the cell surface receptor for fibrinogen. Recent studies by several groups have suggested that GPIIb-IIIa, which is dispersed randomly in the resting cell, undergoes migration leading to receptor clustering after platelet activation. The authors have investigated this activation-dependent relocation of fibrinogen receptors on platelets adherent to a standardized artificial surface. The correlative use of immunogold electron microscopy, ligand-gold binding, and stereo (three-dimensional) electron microscopy (EM) revealed specific localization of fibrinogen and its receptor at points of platelet to platelet interaction. Fibrinogen distribution on the plasma membrane, studied through the use of fibrinogen-gold conjugates with whole-mount adherent platelets, was primarily over the granulomere and at the cell periphery corresponding to sites of platelet-platelet interaction. Compared with the general hyalomere, fibrinogen density over the granulomere and at contact regions was increased 12-fold and 22-fold, respectively, and the specificity of binding at these sites was verified by positive competition with native fibrinogen, one of its degradation products (Fragment D1), and by monoclonal antibodies (HP1-1d and AP-2) specific for GPIIb-IIIa. The distribution of receptor antigens, localized by immunogold EM using antibodies against GPIIb-IIIa, also was localized over the hyalomere, where fibrinogen did not bind. To understand this apparently nonfunctional hyalomere GPIIb-IIIa further, correlative immunocytochemistry was performed using polyclonal and monoclonal antibodies for GPIIb and GPIIIa simultaneously. Colocalization of the antigens was observed consistently over the granulomere and at regions of cell contact, whereas the hyalomere antigens tended to be nonassociated. The studies document GPIIb-IIIa as a function complex at sites of cell interaction where fibrinogen binds.