Rap1-Rac1 Circuits Potentiate Platelet Activation
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.
Methods and 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 novel 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-outand wild-type /EHT platelets; and (2) reduced calcium store release in wild-type /EHT but not double knock-outplatelets. Consistent with the latter finding, we identified 2 pools of Rac1, 1 activated immediately downstream of GPVI and 1 activated downstream of Rap1.
Conclusion—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.
- Received March 31, 2011.
- Accepted October 31, 2011.
- © 2011 American Heart Association, Inc.