title: CalDAG-GEFI is at the nexus of calcium-dependent platelet activation.
abstract: The importance of the second messengers calcium (Ca(2+)) and diacylglycerol (DAG) in platelet signal transduction was established more than 30 years ago. Whereas protein kinase C (PKC) family members were discovered as the targets of DAG, little is known about the molecular identity of the main Ca(2+) sensor(s). We here identify Ca(2+) and DAG-regulated guanine nucleotide exchange factor I (CalDAG-GEFI) as a critical molecule in Ca(2+)-dependent platelet activation. CalDAG-GEFI, through activation of the small GTPase Rap1, directly triggers integrin activation and extracellular signal-regulated kinase-dependent thromboxane A(2) (TxA(2)) release. CalDAG-GEFI-dependent TxA(2) generation provides crucial feedback for PKC activation and granule release, particularly at threshold agonist concentrations. PKC/P2Y12 signaling in turn mediates a second wave of Rap1 activation, necessary for sustained platelet activation and thrombus stabilization. Our results lead to a revised model for platelet activation that establishes one molecule, CalDAG-GEFI, at the nexus of Ca(2+)-induced integrin activation, TxA(2) generation, and granule release. The preferential activation of CalDAG-GEFI over PKC downstream of phospholipase C activation, and the different kinetics of CalDAG-GEFI- and PKC/P2Y12-mediated Rap1 activation demonstrate an unexpected complexity to the platelet activation process, and they challenge the current model that DAG/PKC-dependent signaling events are crucial for the initiation of platelet adhesion.
authors.affiliation: Cardeza Foundation and Department of Medicine, Thomas Jefferson University, Philadelphia, PA 19007, USA.
authors.forename: Lucia
authors.initials: L
authors.lastname: Stefanini
authors.forename: R Claire
authors.initials: RC
authors.lastname: Roden
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Bergmeier
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D001792:Blood Platelets / Q000378:metabolism; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism*; D049109:Cell Proliferation / Q000187:drug effects; D003094:Collagen / Q000378:metabolism; D004075:Diglycerides / Q000378:metabolism*; D005260:Female; D005434:Flow Cytometry; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D015539:Platelet Activation / Q000502:physiology*; D010980:Platelet Membrane Glycoproteins / Q000502:physiology; D012701:Serotonin / Q000378:metabolism; D013928:Thromboxane A2 / Q000378:metabolism; D020827:rap1 GTP-Binding Proteins / Q000378:metabolism
chemical_list: C011439:1,2-diacylglycerol; D004075:Diglycerides; D020662:Guanine Nucleotide Exchange Factors; D010980:Platelet Membrane Glycoproteins; C098341:platelet membrane glycoprotein VI; D012701:Serotonin; D013928:Thromboxane A2; D000244:Adenosine Diphosphate; D003094:Collagen; D020928:Mitogen-Activated Protein Kinases; D020827:rap1 GTP-Binding Proteins; D002118:Calcium