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CIB1 deficiency results in impaired thrombosis: the potential role of CIB1 in outside‐in signaling through integrin αIIbβ3

Summary.  Background: Agonist‐induced inside‐out signaling activates platelet integrin αIIbβ3, rendering it to bind plasma fibrinogen (Fg). Fg binding induces outside‐in signaling that culminates in platelet aggregation, leading to physiological hemostasis and pathological thrombosis. How outside‐in signaling through αIIbβ3 regulates hemostasis and thrombosis is not well understood. We have previously shown that CIB1 is involved in regulating αIIbβ3 function. Objective: To determine the in vivo role of CIB1 in the process of hemostasis and thrombosis. Methods and Results: Genetic ablation of Cib1 significantly increased mouse tail bleeding time. Greater than 50% of the Cib1 null mice showed a rebleeding phenotype. Time taken for complete occlusion of carotid artery upon 10% FeCl3‐induced injury was significantly delayed in the absence of Cib1. This was also associated with unstable thrombus formation. The inside‐out signaling appears normal as ADP‐, collagen‐ and PAR4 peptide‐induced aggregation and fibrinogen binding was unaffected. The absence of Cib1 also affected the ability of platelets to spread on immobilized Fg, but not filopodia formation. Spreading could be restored in Cib1 null platelets by the addition of exogenous ADP. Outside‐in signaling‐dependent tyrosine phosphorylation of the integrin β3 subunit was significantly reduced in the absence of Cib1 as determined by Western blot analysis. Conclusion: Using gene knockout mice, we show for the first time that lack of Cib1 results in impaired thrombosis. CIB1 regulates these processes by affecting platelet spreading, but not platelet filopodia formation. These in vivo and in vitro results clearly show that CIB1 is a key regulator of thrombosis.