Alpha-granule pool of glycoprotein IIb-IIIa in normal and pathologic platelets and megakaryocytes

Abstract

Using an immunogold staining technique and electron microscopy, we investigated the localization of the α-granule pool of glycoprotein (GP) IIb-IIIa in normal platelets and maturing megakaryocytes (MK), in pathologic platelets from a patient with type I Glanzmann's thrombasthenia (GT), and from three patients with the gray platelet syndrome (GPS). In normal resting platelets, GPIIb-IIIa was observed on the plasmatic side of the plasma membrane, the open canicular system (OCS) membranes, and along the internal face of the α-granule membrane. This location was found with three monospecific polyclonal antibodies: one anti-GPIIb-IIIa antibody, the second specific for GPIIb, and the third specific for GPIIIa. After thrombin stimulation, the α-granule labeling disappeared whereas membrane labeling increased. Platelets from GT did not display labeling on plasma membranes, OCS membranes, or α-granule membranes. Platelets from the three patients with GPS displayed intense labeling of the plasma membrane and the OCS membrane, as well as the abnormal small α-granules and along the inside of large vacuoles (which contain the granule membrane protein [GMP]-140). In cultured immature MK from normal progenitors, both peptide components of GPIIb-IIIa appeared in the Golgi saccules and vesicles, and in the small precursors of α-granules, labeling both their membranes and their matrix. It was then observed only on the membrane of the mature MK α-granules, although labeling was less consistent than on the platelet granules. The MK plasma membrane and demarcation membrane system also displayed GPIIb-IIIa labeling. In conclusion, this study demonstrates that GPIIb-IIIa is present on the internal face of the α-granule membranes of platelets (where it appears early during MK maturation) as well as in the abnormal α-granules of gray platelets; it is absent from GT type I platelets. © 1990 by The American Society of Hematology.