Suboptimal Activation of Protease-activated Receptors Enhances 2 1 Integrin-mediated Platelet Adhesion to Collagen

Department of Pathology, Vanderbilt University, Nashville, Tennessee 37232, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 10/2009; 284(50):34640-7. DOI: 10.1074/jbc.M109.020990
Source: PubMed


Thrombin and fibrillar collagen are potent activators of platelets at sites of vascular injury. Both agonists cause platelet shape change, granule secretion, and aggregation to form the primary hemostatic plug. Human platelets express two thrombin receptors, protease-activated receptors 1 and 4 (PAR1 and PAR4) and two collagen receptors, the alpha(2)beta(1) integrin (alpha(2)beta(1)) and the glycoprotein VI (GPVI)/FcRgamma chain complex. Although these receptors and their signaling mechanisms have been intensely studied, it is not known whether and how these receptors cooperate in the hemostatic function of platelets. This study examined cooperation between the thrombin and collagen receptors in platelet adhesion by utilizing a collagen-related peptide (alpha2-CRP) containing the alpha(2)beta(1)-specific binding motif, GFOGER, in conjunction with PAR-activating peptides. We demonstrate that platelet adhesion to alpha2-CRP is substantially enhanced by suboptimal PAR activation (agonist concentrations that do not stimulate platelet aggregation) using the PAR4 agonist peptide and thrombin. The enhanced adhesion induced by suboptimal PAR4 activation was alpha(2)beta(1)-dependent and GPVI/FcRgamma-independent as revealed in experiments with alpha(2)beta(1)- or FcRgamma-deficient mouse platelets. We further show that suboptimal activation of other platelet G(q)-linked G protein-coupled receptors (GPCRs) produces enhanced platelet adhesion to alpha2-CRP. The enhanced alpha(2)beta(1)-mediated platelet adhesion is controlled by phospholipase C (PLC), but is not dependent on granule secretion, activation of alpha(IIb)beta(3) integrin, or on phosphoinositol-3 kinase (PI3K) activity. In conclusion, we demonstrate a platelet priming mechanism initiated by suboptimal activation of PAR4 or other platelet G(q)-linked GPCRs through a PLC-dependent signaling cascade that promotes enhanced alpha(2)beta(1) binding to collagens containing GFOGER sites.

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    • "In platelets, most studies have been performed to understand the molecular mechanism behind integrin a IIb b 3 inside-out activation. Relatively limited information is available about the control of integrin a 2 b 1 (Chen and Kahn, 2003; Jung and Moroi, 2001; Marjoram et al., 2009; Pula and Poole, 2008; Wang et al., 2009) with conflicting observations about the possible contribution of PI3K (Jung and Moroi, 2001; Marjoram et al., 2009), and nothing is known about inside-out regulation of other integrins in platelets. "
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