on November 16, 2006
Published online before print November 16, 2006, doi: 10.1161/01.ATV.0000252826.96134.21
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Submitted on May 31, 2006
Accepted on October 31, 2006
GPVI Potentiation of Platelet Activation by Thrombin and Adhesion Molecules Independent of Src Kinases and Syk
Sascha C. Hughan *;
From the Centre for Cardiovascular Sciences (S.C.H., C.E.H., O.J.T.M., S.P.W.), Institute of Biomedical Research, University of Birmingham, UK; the National Institute for Medical Research (E.S., V.L.J.T.), London, UK; Australian Centre for Blood Diseases (S.C.H.), Monash University, Melbourne, Victoria, Australia; and the Department of Physiology and Biophysics (I.S., J.W.), University of Arkansas for Medical Sciences, Little Rock. Current affiliation for O.J.T.M.: Department of Biomedical Engineering, Oregon Health and Science University, Portland, Ore.
* To whom correspondence should be addressed. E-mail: Sascha.hughan{at}med.monash.edu.au.
Objective--The present study investigates the role of Src and Syk tyrosine kinases in signaling by G-protein coupled and platelet adhesion receptors.
Methods and Results--Using Syk-/- platelets or the Src kinase inhibitor PP2, we demonstrate a critical role for Src and Syk kinases in mediating lamellipodia formation on VWF, collagen, CRP, fibrinogen, and fibronectin. In all cases, the spreading defect was overcome by addition of thrombin. Conversely, platelet aggregation and 
IIb
3 activation induced by thrombin was similar to controls, arguing against a functional role for Src and Syk in IIb3 activation. Unexpectedly, CRP potentiated integrin IIb3 activation and platelet aggregation induced by subthreshold concentrations of thrombin in Syk-/- platelets or in the presence of the Src kinase inhibitor PP2. Potentiation in the presence of PP2 was lost in the absence of FcR
-chain or GPVI confirming that it was mediated through the immunoglobulin receptor. Further delineation of this PP2-resistant synergy revealed that PAR4 could trigger the enhanced response in combination with CRP.
Conclusions--We show that Syk is critical for lamellipodia formation on a range of immobilized proteins but that this can be overcome by addition of thrombin. Further, we reveal a novel role for GPVI in supporting thrombin-induced activation, independent of Syk and Src kinases.
Key words: platelets • GPVI signaling • Src kinases • Syk • thrombin
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