CalDAG-GEFI and protein kinase C represent alternative pathways leading to activation of integrin alphaIIbbeta3 in platelets

Blood. 2008 Sep 1;112(5):1696-703. doi: 10.1182/blood-2008-02-139733. Epub 2008 Jun 10.


Second messenger-mediated inside-out activation of integrin alphaIIbbeta3 is a key step in platelet aggregation. We recently showed strongly impaired but not absent alphaIIbbeta3-mediated aggregation of CalDAG-GEFI-deficient platelets activated with various agonists. Here we further evaluated the roles of CalDAG-GEFI and protein kinase C (PKC) for alphaIIbbeta3 activation in platelets activated with a PAR4 receptor-specific agonist, GYPGKF (PAR4p). Compared with wild-type controls, platelets treated with the PKC inhibitor Ro31-8220 or CalDAG-GEFI-deficient platelets showed a marked defect in aggregation at low (< 1mM PAR4p) but not high PAR4p concentrations. Blocking of PKC function in CalDAG-GEFI-deficient platelets, how-ever, strongly decreased aggregation at all PAR4p concentrations, demonstrating that CalDAG-GEFI and PKC represent separate, but synergizing, pathways important for alphaIIbbeta3 activation. PAR4p-induced aggregation in the absence of CalDAG-GEFI required cosignaling through the Galphai-coupled receptor for ADP, P2Y12. Independent roles for CalDAG-GEFI and PKC/Galphai signaling were also observed for PAR4p-induced activation of the small GTPase Rap1, with CalDAG-GEFI mediating the rapid but reversible activation of this small GTPase. In summary, our study identifies CalDAG-GEFI and PKC as independent pathways leading to Rap1 and alphaIIbbeta3 activation in mouse platelets activated through the PAR4 receptor.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Diphosphate / blood
  • Animals
  • Blood Platelets / drug effects
  • Blood Platelets / metabolism*
  • Blood Platelets / physiology
  • Cell Degranulation / drug effects
  • GTP-Binding Protein alpha Subunits, Gi-Go / blood
  • Guanine Nucleotide Exchange Factors / blood*
  • Guanine Nucleotide Exchange Factors / deficiency
  • Guanine Nucleotide Exchange Factors / genetics
  • Kinetics
  • Mice
  • Mice, Knockout
  • Models, Biological
  • Oligopeptides / pharmacology
  • Platelet Aggregation / drug effects
  • Platelet Aggregation / physiology
  • Platelet Glycoprotein GPIIb-IIIa Complex / metabolism*
  • Protein Kinase C / antagonists & inhibitors
  • Protein Kinase C / blood*
  • Receptors, Thrombin / blood
  • Receptors, Thrombin / drug effects
  • Signal Transduction
  • rap1 GTP-Binding Proteins / blood


  • GYPGKF-NH(2)
  • Guanine Nucleotide Exchange Factors
  • Oligopeptides
  • Platelet Glycoprotein GPIIb-IIIa Complex
  • Rasgrp2 protein, mouse
  • Receptors, Thrombin
  • Adenosine Diphosphate
  • Protein Kinase C
  • GTP-Binding Protein alpha Subunits, Gi-Go
  • rap1 GTP-Binding Proteins
  • protease-activated receptor 4