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Because CIB1 is one of a few proteins known to bind directly to the αIIb cytoplasmic tail, we hypothesized that CIB1 may modulate platelet αIIbβ3 activation.
To determine whether CIB1 affects αIIbβ3 activation, we used differentiated megakaryocytes from murine bone marrow because megakaryocytes, unlike platelets, are amenable to direct genetic manipulation.
To address the role of CIB1 in αIIbβ3 activation, a well-characterized megakaryocyte model system (Shiraga et al., 1999; Shattil and Leavitt, 2001; Bertoni et al., 2002) was used.
Stimulation of mature murine megakaryocytes with protease-activated receptor 4 activating peptide (PAR4P) significantly increased fibrinogen binding over basal levels to unstimulated megakaryocytes (agonist-induced binding is shown as percent over basal binding, which was subtracted from total binding).
In response to agonist stimulation, the αIIbβ3 integrin on platelets is converted to an active conformation that binds fibrinogen and mediates platelet aggregation.
In addition, expression levels of αIIbβ3 and basal fibrinogen binding to unstimulated megakaryocytes were comparable in megakaryocytes expressing CIB1 F173A-EGFP versus CIB1-EGFP (Fig. These data suggest that a direct interaction between CIB1 and the αIIb tail is critical for suppression of αIIbβ3 activation.
To further determine whether CIB1 suppresses integrin activation by a direct or indirect mechanism, we tested its ability to suppress activation of αV integrins because we previously determined that CIB1 does not interact with the αV cytoplasmic tail (Naik et al., 1997; Barry et al., 2002) and because megakaryocytes express the αV integrin subunit (Fig. We found that neither CIB1-EGFP nor CIB1 F173A-EGFP had an effect on αV integrin activation as detected with WOW-1, a m Ab that selectively recognizes activated αVβ3 and to a lesser extent, activated αVβ5 (Pampori et al., 1999), compared with untransduced megakaryocytes or megakaryocytes expressing EGFP alone (Fig. These results suggest that CIB1 selectively inhibits the activation of αIIbβ3, most likely via a direct interaction with the integrin.
The PAR4P-induced fibrinogen binding was completely blocked by an anti-αIIbβ3 function-blocking m Ab, 1B5 (Fig.
S1 A, available at in agreement with Shiraga et al.