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Osteonectin is a 32-Kd phosphoglycoprotein originally described in bone but also found in platelets. Platelet and bone osteonectin are different both structurally and immunologically. We have previously shown that platelet osteonectin, by binding to thrombospondin, is involved in the secretion-dependent phase of the platelet aggregation process. In this study, we used antiosteonectin antibodies in combination with immunogold labeling to investigate by electron microscopy the fine localization of osteonectin within normal and gray platelets. Using both a polyclonal and monoclonal antibody ON3, osteonectin was specifically located at the internal face of alpha-granule membranes within normal platelets. Osteonectin was not distributed within all alpha-granules, probably because of its low platelet content. In addition, using immunofluorescence, osteonectin could also be detected in immature and mature megakaryocytes with a granular pattern of staining, suggesting that osteonectin is synthesized by megakaryocytes. Using platelets from two patients with gray platelet syndrome, osteonectin was absent within all abnormal small alpha-granules, but was detected in some rare normal-sized alpha-granules. In separate double-label studies, thrombospondin and von Willebrand factor did not colocalize with osteonectin in resting platelets. However, osteonectin was located at the inner face of the alpha-granules, as it is for alpha-granule membrane protein GMP-140 and glycoprotein IIb-IIIa. These results, taken together with the fact that monoclonal antibodies to osteonectin bind only to the surface of activated platelets, suggest that platelet osteonectin is redistributed to the cell surface during fusion of alpha-granule membranes with the plasma membrane.

Localization of platelet osteonectin at the internal face of the alpha- granule membranes in platelets and megakaryocytes