Submitted May 9, 2006
Accepted September 9, 2006
ADAP is required for normal 
IIb
3 activation by VWF/GP Ib-IX-V and other agonists
Ana Kasirer-Friede*, Paul Barry Moran, Jennifer Nagrampa-Orje, Ken Swanson, Zaverio M. Ruggeri, Burkhart Schraven, Benjamin G. Neel, Gary A. Koretzky, and Sanford J. Shattil
Dept of Medicine, University of California San Diego, La Jolla, CA, United States
Dept of Molecular & Experimental Medicine, The Scripps Research Institute, La Jolla, CA, United States
Dept of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, United States
Institute of Immunology, University of Magdeburg, Magdeburg, Germany
Abramson Family Cancer Research Institute, University of Pennsylvania School of Medicine, Philadelphia, PA, United States
* Corresponding author; email: akasirer{at}ucsd.edu.
Interaction between von Willebrand factor (VWF) and platelet GP Ib-IX-V is required for hemostasis, in part because intracellular signals from VWF/GP Ib-IX-V activate the ligand-binding function of integrin 
IIb
3. Since they also induce tyrosine phosphorylation of the ADAP adapter, we investigated ADAP's role in GP Ib-IX-V signal transduction. Fibrinogen or ligand-mimetic POW-2 Fab binding to IIb3 was stimulated by adhesion of ADAP+/+ murine platelets to dimeric VWF A1A2, but was significantly reduced in ADAP-/- platelets (P < 0.01). IIb3 activation by ADP or a Par4 thrombin receptor agonist was also decreased in ADAP-/- platelets. ADAP stabilized the expression of another adapter, SKAP-HOM, via interaction with latter's SH3 domain. However, no abnormalities in IIb3 activation were observed in SKAP-HOM-/- platelets, which express normal ADAP levels, further implicating ADAP as a modulator of IIb3 function. Under shear flow conditions over a combined matrix of VWF A1A2 and fibronectin to test interactions involving GP Ib-IX-V and IIb3, respectively, ADAP-/- platelets displayed reduced IIb3-dependent stable adhesion. Furthermore, ADAP-/- mice demonstrated increased re-bleeding from tail wounds. These studies establish ADAP as a necessary component of inside-out signaling pathways that couple GP Ib-IX-V and other platelet agonist receptors to IIb3 activation.
