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Thromb Haemost. 2010 May;103(5):1044-52. doi: 10.1160/TH09-09-0644. Epub 2010 Apr 13.

The majority of circulating platelet-derived microparticles fail to bind annexin V, lack phospholipid-dependent procoagulant activity and demonstrate greater expression of glycoprotein Ib.

Author information

1
Department of Haematology and Haematological Stem Cell Transplantation, St Vincent's Hospital, Sydney, New South Wales, Australia. dconnor@stvincents.com.au

Abstract

It has been widely accepted that microparticles expose phosphatidylserine which in turn binds annexin V. It was the objective of this study to compare the antigenic characteristics and phospholipid-dependent procoagulant activity of annexin V positive and -negative subpopulations of platelet-derived microparticles. Annexin V positive and -negative microparticles were identified and characterised using flow cytometry and procoagulant activity was measured by a phospholipid-dependent assay (XACT). In unstimulated platelet-poor plasma, 80% of platelet-derived microparticles failed to bind annexin V. Varying the assay constituents (buffer, calcium and annexin V concentration) did not alter annexin V binding. The proportion of microparticles that bound annexin V was dependent upon the agonist, with physiological agonists such as collagen resulting in fewer annexin V binding microparticles than non-physiological agonists such as ionophore. CD42b (glycoprotein Ib) expression was significantly decreased and CD62p and CD63 expression were significantly increased in annexin V positive compared to annexin V negative subpopulations. There was no significant difference in CD41, CD61, CD42a and CD40L expression between annexin V positive and -negative subpopulations. A significant correlation between annexin V binding and XACT was found (p=0.033). Annexin V inhibited greater than 95% of phospholipid activity, suggesting that annexin V binding was a true reflection of procoagulant activity. The majority of platelet-derived microparticles in unstimulated plasma failed to bind annexin V and showed significantly increased levels of CD42b compared to annexin V positive events. Phospholipid-dependent procoagulant activity is limited to the annexin V positive subpopulation and is agonist-dependent. The significance of annexin V negative microparticles is unclear, however, it is possible that they possess other activities aside from procoagulant phospholipid activity.

PMID:
20390225
DOI:
10.1160/TH09-09-0644
[Indexed for MEDLINE]

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