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Rheumatology Advance Access originally published online on July 4, 2006
Rheumatology 2007 46(1):81-86; doi:10.1093/rheumatology/kel200
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© The Author 2006. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Annexin A5 binding to giant phospholipid vesicles is differentially affected by anti-ß2-glycoprotein I and anti-annexin A5 antibodies

N. Gaspersic, A. Ambrozic, B. Bozic, J. Majhenc1, S. Svetina1 and B. Rozman

Department of Rheumatology, University Medical Centre and 1Institute of Biophysics, School of Medicine, University of Ljubljana, Ljubljana, Slovenia

Correspondence to: N. Gaspersic, University Medical Centre, Department of Rheumatology, Vodnikova 62, SI-1000 Ljubljana, Slovenia. E-mail: ngaspersic{at}yahoo.com


  Abstract

Objectives. Anti-phospholipid antibodies have been recognized to play a role in vascular thrombosis and pregnancy morbidity. They were first thought to be directed to phospholipids, but it is now known that the majority of pathogenic antibodies recognizes epitopes on phospholipid-binding plasma proteins such as ß2-glycoprotein I (ß2GPI) or possibly also annexin A5 (ANXA5). The mechanism of their prothrombotic action is still not completely understood. The aim of the present study was to observe the effect of antibodies against ANXA5 (aANXA5) and antibodies against ß2GPI (aß2GPI) on the binding of ANXA5 to the negatively charged phospholipid membrane.

Methods. Giant phospholipid vesicles (GPVs) were used as a simple model of the membrane surface. GPVs composed of phosphatidylserine and phosphatidylcholine were produced in an aqueous medium. A single GPV was transferred to the solution containing ANXA5 conjugated with Alexa Fluor 488 (FANXA5) and (i) aANXA5 or aß2GPI and (ii) different concentrations of aß2GPI together with ß2GPI. The emission of the fluorescent light from the GPV surface, as the result of FANXA5 binding, was measured.

Results. ß2GPI together with aß2GPI reduced the binding of FANXA5 to GPVs. On the contrary, aANXA5 enhanced the binding of ANXA5 to the GPV surface.

Conclusions. Our results point to the competition between FANXA5 and complexes of ß2GPI–aß2GPI for the same binding sites and therefore support the hypothesis of the disruption of the ANXA5 protective shield on procoagulant phospholipid surface. The influence of increased cell surface ANXA5 concentration in the presence of aANXA5 on coagulation needs to be further studied.

KEY WORDS: Annexin A5, Anti-annexin A5, Anti-ß2-glycoprotein I, Giant phospholipid vesicles


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