ALPHA-GRANULE MEMBRANE MIRRORS THE PLATELET PLASMA-MEMBRANE AND CONTAINS THE GLYCOPROTEIN-IB, GLYCOPROTEIN-IX, AND GLYCOPROTEIN-V

We have recently shown that several components from the platelet plasm a membrane were also present at different rates in the alpha-granule m embrane. This is the case for the glycoprotein (GP) IIb-IIIa (CD41). C D36, CD9, PECAM1. and RapIb, while the GPIb-IX-V complex was considere d to escape the rule. In this investigation, we studied the subcellula r localization of GPIb, GPIX, and GPV in the resting platelets of norm al subjects, patients with Bernard-Soulier syndrome, patients with Gra y platelet syndrome, and human cultured megakaryocytes. Ultra-thin sec tions of the cells were labeled with antibodies directed against glyco calicin, GPIb, GPIX, and GPV. We have shown that a significant and rep roducible labeling for the three GPs was associated with the alpha-gra nule membrane, accounting for approximately 10% of the total labeling. Furthermore, GPIb labeling appears colocalized with its alpha-granule -associated ligand, von Willebrand factor (VWF). After thrombin activa tion, vWF remained close to the limiting membrane of the open canalicu lar system (OCS), suggesting an early association of both receptor and ligand. Plasma membrane and alpha-granule labeling was virtually abse nt from the Bernard-Soulier platelets (characterized by a GPIb deficie ncy), thus proving the specificity of the reaction. In Gray platelets (storage granule deficiency syndrome), the small residual alpha-granul es were also occasionally labeled for GPIb, GPIX, and GPV. Cultured me gakaryocytes that displayed the classical GPIb distribution, eg, demar cation and plasma membranes, exhibited also a discrete labeling associ ated to the alpha-granules. In conclusion, this study shows that, even ly for these three GPs, the alpha-granule membrane mirrors the plasma membrane composition. This might occur through an endocytotic process affecting each plasma membrane protein to a different extent and could have a physiologic relevance in further presentation of a receptor bo und to its alpha-granule ligand to the platelet surface. (C) 1996 by T he American Society of Hematology.