title: Rap1-Rac1 circuits potentiate platelet activation.
abstract: OBJECTIVE
  The goal of this study was to investigate the potential crosstalk between Rap1 and Rac1, 2 small GTPases central to platelet activation, particularly downstream of the collagen receptor GPVI.
  
  
  RESULTS
  We compared the activation response of platelets with impaired Rap signaling (double knock-out; deficient in both the guanine nucleotide exchange factor, CalDAG-GEFI, and the Gi-coupled receptor for ADP, P2Y12), to that of wild-type platelets treated with a small-molecule Rac inhibitor, EHT 1864 (wild-type /EHT). We found that Rac1 is sequentially activated downstream of Rap1 on stimulation via GPVI. In return, Rac1 provides important feedback for both CalDAG-GEFI- and P2Y12-dependent activation of Rap1. When analyzing platelet responses controlled by Rac1, we observed (1) impaired lamellipodia formation, clot retraction, and granule release in both double knock-out and EHT 1864-treated wild-type platelets; and (2) reduced calcium store release in EHT 1864-treated wild-type but not double knock-out platelets. Consistent with the latter finding, we identified 2 pools of Rac1, one activated immediately downstream of GPVI and 1 activated downstream of Rap1.
  
  
  CONCLUSIONS
  We demonstrate important crosstalk between Rap1 and Rac1 downstream of GPVI. Whereas Rap1 signaling directly controls sustained Rac1 activation, Rac1 affects CalDAG-GEFI- and P2Y12-dependent Rap1 activation via its role in calcium mobilization and granule/ADP release, respectively.
authors.affiliation: Dept. of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, 98 Manning Drive, 306a Mary Ellen Jones Building, Chapel Hill, NC 27599, USA.
authors.forename: Lucia
authors.initials: L
authors.lastname: Stefanini
authors.forename: Yacine
authors.initials: Y
authors.lastname: Boulaftali
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Ouellette
authors.forename: Michael
authors.initials: M
authors.lastname: Holinstat
authors.forename: Laurent
authors.initials: L
authors.lastname: Désiré
authors.forename: Bertrand
authors.initials: B
authors.lastname: Leblond
authors.forename: Patrick
authors.initials: P
authors.lastname: Andre
authors.forename: Pamela B
authors.initials: PB
authors.lastname: Conley
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Bergmeier
mesh_terms: D000818:Animals; D025461:Feedback, Physiological / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D023421:Models, Animal; D015539:Platelet Activation / Q000502:physiology*; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism; D058925:Receptors, Purinergic P2Y12 / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D020830:rac1 GTP-Binding Protein / Q000378:metabolism*; D020827:rap1 GTP-Binding Proteins / Q000378:metabolism*
chemical_list: D020662:Guanine Nucleotide Exchange Factors; C548577:P2ry12 protein, mouse; D010980:Platelet Membrane Glycoproteins; C485927:Rasgrp2 protein, mouse; D058925:Receptors, Purinergic P2Y12; C098341:platelet membrane glycoprotein VI; D020830:rac1 GTP-Binding Protein; D020827:rap1 GTP-Binding Proteins
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