CalDAG-GEFI and protein kinase C represent alternative pathways leading to activation of integrin alphaIIbbeta3 in platelets.
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.
Immune Disease Institute, Harvard Medical School, Boston, MA, USA.
Stephen M
SM
Cifuni
Denisa D
DD
Wagner
Wolfgang
W
Bergmeier
D000244:Adenosine Diphosphate / Q000097:blood; D000818:Animals; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D015550:Cell Degranulation / Q000187:drug effects; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000097:blood; D020662:Guanine Nucleotide Exchange Factors / Q000097:blood* / Q000172:deficiency / Q000235:genetics; D007700:Kinetics; D051379:Mice; D018345:Mice, Knockout; D008954:Models, Biological; D009842:Oligopeptides / Q000494:pharmacology; D010974:Platelet Aggregation / Q000187:drug effects / Q000502:physiology; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000097:blood*; D018179:Receptors, Thrombin / Q000097:blood / Q000187:drug effects; D015398:Signal Transduction; D020827:rap1 GTP-Binding Proteins / Q000097:blood
C482002:GYPGKF-NH(2); D020662:Guanine Nucleotide Exchange Factors; D009842:Oligopeptides; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; C485927:Rasgrp2 protein, mouse; D018179:Receptors, Thrombin; D000244:Adenosine Diphosphate; D011493:Protein Kinase C; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D020827:rap1 GTP-Binding Proteins; C112908:protease-activated receptor 4