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Expression of platelet glycoprotein Ib by cultured human megakaryocytes: ultrastructural localization and
N Debili1, N Kieffer, M Nakazawa
1INSERM U91, Hôpital Henri Mondor, Créteil, France.
Abstract:
Glycoprotein Ib (GPIb), the receptor for von Willebrand factor, is a two-chain member constituent of the platelet/megakaryocytic lineage. Studies on its expression have been hampered by the difficulties in obtaining purified megakaryocytes in a sufficient number. We report a suspension liquid culture procedure that allowed isolation of more than 1 x 10(6) megakaryocytes with a purity ranging from 3% to 88% from the blood of patients with chronic myeloid leukemia, from fetal liver or from normal human bone marrow. GPIb was detected on the plasma membrane of all maturing megakaryocytes and also of promegakaryoblasts devoid of demarcation membranes. GPIb was detected on demarcation membranes of maturing megakaryocytes but was absent from all other organelles, including alpha granules. Biosynthesis of 35S-methionine labeled megakaryocytes showed that GPIb with similar electrophoretic mobility to the platelet molecule was synthesized and that it was also composed of two chains, since its molecular weight shifted in reducing conditions from 170 Kd to 145 Kd. The beta chain remained undetectable after methionine metabolic labeling, but it was immunoprecipitated after 3H-leucine metabolic labeling, confirming that this subunit is devoid of methionine. GPIb was associated with GPIX, as it is in platelets, since anti-GPIb antibodies coprecipitated a 17 Kd polypeptide.
Insights
Researchers developed a novel method to isolate megakaryocytes, enabling the study of Glycoprotein Ib (GPIb) expression. This receptor is crucial for platelet function and is present on maturing megakaryocytes and their precursors.
Area of Science:
- Hematology
- Cell Biology
- Molecular Biology
Background:
- Glycoprotein Ib (GPIb) is a vital receptor for von Willebrand factor, essential for platelet adhesion.
- Previous studies on GPIb expression were limited by challenges in obtaining sufficient pure megakaryocytes.
Purpose of the Study:
- To establish a reliable method for megakaryocyte isolation and purification.
- To investigate the expression and synthesis of GPIb in megakaryocytes.
Main Methods:
- Developed a suspension liquid culture technique for megakaryocyte isolation.
- Utilized immunofluorescence and metabolic labeling (35S-methionine, 3H-leucine) to detect and characterize GPIb.
- Employed immunoprecipitation and SDS-PAGE to analyze GPIb structure and association with GPIX.
Main Results:
- Successfully isolated over 1 x 10(6) megakaryocytes with varying purity from diverse sources.
- Detected GPIb on the plasma membrane and demarcation membranes of maturing megakaryocytes and promegakaryoblasts.
- Confirmed GPIb biosynthesis as a two-chain molecule, associated with GPIX, and localized to specific cellular compartments.
Conclusions:
- The developed culture method effectively enables megakaryocyte isolation for GPIb research.
- GPIb is expressed early in megakaryocyte development and is correctly assembled with GPIX.