Plasma membrane GPIIb/IIIa. Evidence for a cycling receptor pool.
The author used immunofluorescence and digital image processing to investigate the dynamic distribution of GPIIb/IIIa in living platelets. Resting cells were incubated with AP-2, a complex-specific, monoclonal, anti-GPIIb/IIIa antibody. Examination of intact cells demonstrated a rim pattern for GPIIb/IIIa consistent with a surface localization. Permeabilization revealed a time-dependent increase in the labeling of apparent intracellular vacuoles. This pattern is distinct from the "patch-cap" pattern observed when unfixed platelets were incubated with fluoresceinated concanavalin A. Additionally, labeling of this vacuolar pool of GPIIb/IIIa was inhibited by treatment with 2% sodium azide or by incubation at 4 degrees C. Identical staining patterns were obtained with Fab fragments of AP-2. Ultrastructural examination confirmed the presence of labeled intracellular vacuolar structures. Parallel studies performed with AP-1, a monoclonal anti-GPIb antibody, failed to demonstrate internalization of GPIb. Finally, thrombin stimulation of resting platelets, which had been preincubated with AP-2, resulted in the clearing of this newly internalized pool of GPIIb/IIIa; presumably via translocation to the surface. These data suggest the presence of an actively cycling pool of GPIIb/IIIa that has not been described previously. The dynamic distribution of this pool may be important in the regulation of platelet adhesiveness.
University of Chicago Medical Center, Department of Medicine, Illinois 60637.
J D
JD
Wencel-Drake
D000906:Antibodies / Q000032:analysis; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D004705:Endocytosis / Q000187:drug effects; D005455:Fluorescent Antibody Technique; D006801:Humans; D008854:Microscopy, Electron; D010980:Platelet Membrane Glycoproteins / Q000032:analysis / Q000276:immunology / Q000378:metabolism*; D013917:Thrombin / Q000494:pharmacology
D000906:Antibodies; D010980:Platelet Membrane Glycoproteins; D013917:Thrombin