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Localization of platelet osteonectin at the internal face of the alpha-
granule membranes in platelets and megakaryocytes
J Breton-Gorius, P Clezardin, J Guichard, N Debili, L Malaval, W Vainchenker, EM Cramer and PD Delmas
Unite INSERM U. 91, Hopital Henri Mondor, Creteil, France.
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.
Volume 79,
Issue 4,
pp. 936-941,
02/15/1992
Copyright © 1992 by The American Society of Hematology
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