Platelet activation by dimeric beta2-glycoprotein I requires signaling via both glycoprotein Ibalpha and apolipoprotein E receptor 2'.
BACKGROUND
Dimerization of beta(2)-glycoprotein I (beta(2)-GPI) by autoantibodies is thought to trigger the clinical manifestations observed in the antiphospholipid syndrome. Arterial thrombosis, a frequently occurring clinical manifestation of the antiphospholipid syndrome, is a process in which platelets play a crucial role. Previous work has shown that binding of dimeric beta(2)-GPI to the platelet receptors apolipoprotein E receptor 2' (ApoER2') and glycoprotein Ibalpha (GPIbalpha) mediates increased platelet activation in an in vitro thrombosis model.


OBJECTIVE
The individual roles of ApoER2' and GPIbalpha in mediating platelet activation by dimeric beta(2)-GPI has hitherto been unclear. In this study, we have determined the roles of either receptor in platelet activation by dimeric beta(2)-GPI.


METHODS
Platelet activation by dimeric beta(2)-GPI was studied under conditions of flow. Intracellular signaling induced by dimeric beta(2)-GPI was subsequently analyzed by means of sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analysis.


RESULTS
The increase in platelet deposition onto a fibronectin surface under conditions of flow by dimeric beta(2)-GPI was completely abolished by inhibition of the interaction of dimeric beta(2)-GPI with either GPIbalpha or ApoER2'. Upon platelet stimulation with dimeric beta(2)-GPI, GPIbalpha translocated to the cytoskeleton via the scaffold protein 14-3-3zeta. Concomitantly, ApoER2' dissociated from the adapter protein Disabled1, presumably through phosphorylation of the cytoplasmic tail. Inhibition of one process could not inhibit the other.


CONCLUSIONS
We show that dimeric beta(2)-GPI signals via two distinct pathways in platelets, both of which are required for platelet activation. Abrogation of either signal results in loss of activation.
Urbanus
R T
RT
Department of Clinical Chemistry and Haematology, University Medical Centre Utrecht, Utrecht, The Netherlands.
Pennings
M T T
MT
Derksen
R H W M
RH
de Groot
P G
PG
D048948:14-3-3 Proteins / Q000097:blood; D048868:Adaptor Proteins, Signal Transducing / Q000097:blood; D000595:Amino Acid Sequence; D016736:Antiphospholipid Syndrome / Q000097:blood / Q000276:immunology; D001323:Autoantibodies / Q000097:blood; D001693:Biological Transport, Active; D001792:Blood Platelets / Q000187:drug effects / Q000276:immunology / Q000378:metabolism; D019281:Dimerization; D006801:Humans; D066298:In Vitro Techniques; D026502:LDL-Receptor Related Proteins; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000097:blood; D015539:Platelet Activation / Q000187:drug effects* / Q000502:physiology*; D010973:Platelet Adhesiveness / Q000187:drug effects / Q000502:physiology; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000378:metabolism*; D018110:Receptors, Lipoprotein / Q000097:blood* / Q000235:genetics; D015398:Signal Transduction; D053482:beta 2-Glycoprotein I / Q000097:blood* / Q000737:chemistry / Q000276:immunology / Q000494:pharmacology*
D048948:14-3-3 Proteins; D048868:Adaptor Proteins, Signal Transducing; D001323:Autoantibodies; C115785:DAB1 protein, human; D026502:LDL-Receptor Related Proteins; D009419:Nerve Tissue Proteins; D019038:Platelet Glycoprotein GPIb-IX Complex; D018110:Receptors, Lipoprotein; D053482:beta 2-Glycoprotein I; C098773:low density lipoprotein receptor-related protein 8