title: Distinct roles for Rap1b protein in platelet secretion and integrin αIIbβ3 outside-in signaling.
abstract: Rap1b is activated by platelet agonists and plays a critical role in integrin α(IIb)β(3) inside-out signaling and platelet aggregation. Here we show that agonist-induced Rap1b activation plays an important role in stimulating secretion of platelet granules. We also show that α(IIb)β(3) outside-in signaling can activate Rap1b, and integrin outside-in signaling-mediated Rap1b activation is important in facilitating platelet spreading on fibrinogen and clot retraction. Rap1b-deficient platelets had diminished ATP secretion and P-selectin expression induced by thrombin or collagen. Importantly, addition of low doses of ADP and/or fibrinogen restored aggregation of Rap1b-deficient platelets. Furthermore, we found that Rap1b was activated by platelet spreading on immobilized fibrinogen, a process that was not affected by P2Y(12) or TXA(2) receptor deficiency, but was inhibited by the selective Src inhibitor PP2, the PKC inhibitor Ro-31-8220, or the calcium chelator demethyl-1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis. Clot retraction was abolished, and platelet spreading on fibrinogen was diminished in Rap1b-deficient platelets compared with wild-type controls. The defects in clot retraction and spreading on fibrinogen of Rap1b-deficient platelets were not rescued by addition of MnCl(2), which elicits α(IIb)β(3) outside-in signaling in the absence of inside-out signaling. Thus, our results reveal two different activation mechanisms of Rap1b as well as novel functions of Rap1b in platelet secretion and in integrin α(IIb)β(3) outside-in signaling.
authors.affiliation: Division of Cardiovascular Medicine, The Gill Heart Institute, University of Kentucky, Lexington, Kentucky 40536, USA.
authors.forename: Guoying
authors.initials: G
authors.lastname: Zhang
authors.forename: Binggang
authors.initials: B
authors.lastname: Xiang
authors.forename: Shaojing
authors.initials: S
authors.lastname: Ye
authors.forename: Magdalena
authors.initials: M
authors.lastname: Chrzanowska-Wodnicka
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Morris
authors.forename: T Kent
authors.initials: TK
authors.lastname: Gartner
authors.forename: Sidney W
authors.initials: SW
authors.lastname: Whiteheart
authors.forename: Gilbert C
authors.initials: GC
authors.lastname: White
authors.forename: Susan S
authors.initials: SS
authors.lastname: Smyth
authors.forename: Zhenyu
authors.initials: Z
authors.lastname: Li
mesh_terms: D000255:Adenosine Triphosphate / Q000235:genetics / Q000378:metabolism; D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D003018:Clot Retraction / Q000187:drug effects / Q000502:physiology*; D004789:Enzyme Activation / Q000187:drug effects / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005340:Fibrinogen / Q000378:metabolism; D007211:Indoles / Q000494:pharmacology; D051379:Mice; D008817:Mice, Mutant Strains; D019007:P-Selectin / Q000235:genetics / Q000378:metabolism; D010973:Platelet Adhesiveness / Q000187:drug effects / Q000502:physiology*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000235:genetics / Q000378:metabolism*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D011743:Pyrimidines / Q000494:pharmacology; D058925:Receptors, Purinergic P2Y12 / Q000235:genetics / Q000378:metabolism; D044244:Receptors, Thromboxane A2, Prostaglandin H2 / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D020668:rap GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: C412373:AG 1879; D002614:Chelating Agents; D004791:Enzyme Inhibitors; D007211:Indoles; D019007:P-Selectin; C548577:P2ry12 protein, mouse; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D011743:Pyrimidines; D058925:Receptors, Purinergic P2Y12; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D000255:Adenosine Triphosphate; D005340:Fibrinogen; D011493:Protein Kinase C; C492136:Rap1b protein, mouse; D020668:rap GTP-Binding Proteins; C064758:Ro 31-8220