title: Platelet adhesion to dimeric beta-glycoprotein I under conditions of flow is mediated by at least two receptors: glycoprotein Ibalpha and apolipoprotein E receptor 2'.
abstract: BACKGROUND
  The major antigen implicated in the antiphospholipid syndrome is beta2-glycoprotein I (beta2GPI). Dimerized beta2GPI binds to apolipoprotein E receptor 2' (apoER2') on platelets and increases platelet adhesion to collagen under conditions of flow.
  
  
  OBJECTIVE
  To investigate whether the interaction between dimerized beta2GPI and platelets is sufficiently strong to resist shear stresses.
  
  
  METHODS
  We studied the interaction of platelets with immobilized dimerized beta2GPI under conditions of flow, and further analyzed the interaction using surface plasmon resonance and solid phase immunoassays.
  
  
  RESULTS
  We found that dimerized beta2GPI supports platelet adhesion and aggregate formation under venous flow conditions. Adhesion of platelets to dimerized beta2GPI was completely inhibited by the addition of soluble forms of both apoER2' and GPIbalpha, and the addition of receptor-associated protein and the removal of GPIbalpha from the platelet surface. GPIbalpha co-precipitated with apoER2', suggesting the presence of complexes between GPIbalpha and apoER2' on platelet membranes. The interaction between GPIbalpha and dimeric beta2GPI was of intermediate affinity (Kd = 180 nM) and Zn2+, but not Ca2+-dependent. Deletion of domain V from dimeric beta2GPI strongly reduced its binding to both GPIbalpha and apoER2'. Antibodies that inhibit the binding of thrombin to GPIbalpha inhibited platelet adhesion to dimeric beta2GPI completely, while antibodies blocking the binding of von Willebrand factor to GPIbalpha had no effect. Dimeric beta2GPI showed reduced binding to low-sulfated GPIbalpha compared to the fully sulfated form.
  
  
  CONCLUSIONS
  We show that platelets adhere to dimeric beta2GPI under both arterial and venous shear stresses. Platelets adhere via two receptors: GPIbalpha and apoER2'. These receptors are present in a complex on the platelet surface.
authors.affiliation: Laboratory of Clinical Chemistry and Haematology, University Medical Centre Utrecht, Utrecht, the Netherlands.
authors.forename: M T T
authors.initials: MT
authors.lastname: Pennings
authors.forename: R H W M
authors.initials: RH
authors.lastname: Derksen
authors.forename: M
authors.initials: M
authors.lastname: van Lummel
authors.forename: J
authors.initials: J
authors.lastname: Adelmeijer
authors.forename: K
authors.initials: K
authors.lastname: VanHoorelbeke
authors.forename: R T
authors.initials: RT
authors.lastname: Urbanus
authors.forename: T
authors.initials: T
authors.lastname: Lisman
authors.forename: P G
authors.initials: PG
authors.lastname: de Groot
mesh_terms: D001792:Blood Platelets / Q000378:metabolism; D003094:Collagen / Q000378:metabolism; D019281:Dimerization; D006801:Humans; D007118:Immunoassay; D026502:LDL-Receptor Related Proteins; D046912:Multiprotein Complexes / Q000378:metabolism / Q000502:physiology; D010477:Perfusion; D010973:Platelet Adhesiveness; D010974:Platelet Aggregation; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism; D018110:Receptors, Lipoprotein / Q000378:metabolism*; D013314:Stress, Mechanical; D020349:Surface Plasmon Resonance; D053482:beta 2-Glycoprotein I / Q000378:metabolism*
chemical_list: D026502:LDL-Receptor Related Proteins; D046912:Multiprotein Complexes; D019038:Platelet Glycoprotein GPIb-IX Complex; D011956:Receptors, Cell Surface; D018110:Receptors, Lipoprotein; D053482:beta 2-Glycoprotein I; C023592:glycocalicin; C098773:low density lipoprotein receptor-related protein 8; D003094:Collagen