title: Suboptimal activation of protease-activated receptors enhances alpha2beta1 integrin-mediated platelet adhesion to collagen.
abstract: 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.
authors.affiliation: Department of Pathology, Vanderbilt University, Nashville, Tennessee 37232, USA.
authors.forename: Robin J
authors.initials: RJ
authors.lastname: Marjoram
authors.forename: Bryan
authors.initials: B
authors.lastname: Voss
authors.forename: Yumei
authors.initials: Y
authors.lastname: Pan
authors.forename: S Kent
authors.initials: SK
authors.lastname: Dickeson
authors.forename: Mary M
authors.initials: MM
authors.lastname: Zutter
authors.forename: Heidi E
authors.initials: HE
authors.lastname: Hamm
authors.forename: Samuel A
authors.initials: SA
authors.lastname: Santoro
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D003094:Collagen / Q000378:metabolism*; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism; D006801:Humans; D038982:Integrin alpha2beta1 / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D010455:Peptides / Q000235:genetics / Q000378:metabolism; D010973:Platelet Adhesiveness / Q000502:physiology*; D010974:Platelet Aggregation; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism; D051381:Rats; D044463:Receptor, PAR-1 / Q000378:metabolism*; D038981:Receptors, Collagen / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism; D017452:Receptors, IgG / Q000235:genetics / Q000378:metabolism; D018179:Receptors, Thrombin / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D013917:Thrombin / Q000378:metabolism; D010738:Type C Phospholipases / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D038982:Integrin alpha2beta1; D010455:Peptides; D010980:Platelet Membrane Glycoproteins; D044463:Receptor, PAR-1; D038981:Receptors, Collagen; D043562:Receptors, G-Protein-Coupled; D017452:Receptors, IgG; D018179:Receptors, Thrombin; C435908:collagen-related peptide; C098341:platelet membrane glycoprotein VI; D003094:Collagen; D010738:Type C Phospholipases; D013917:Thrombin; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; C112908:protease-activated receptor 4