Characterization of a Novel Integrin Binding Protein, VPS33B, Which is Important for Platelet Activation and in vivo Thrombosis and Hemostasis
Background—Integrins are heterodimeric (α/β) membrane proteins that play fundamental roles in many biological processes, e.g. cell adhesion and spreading, which are important for platelet function and hemostasis. The molecular mechanism that regulates integrin activation is not completely understood.
Methods and Results—Here, we show that VPS33B, a member of the Sec1/Munc18 (SM) family and a component of the Vps-C tethering complexes, binds directly to the integrin β subunit. Overexpression of VPS33B in CHO cells potentiated αIIbβ3 outside-in signaling but not inside-out signaling. Platelets, from megakaryocyte- and platelet-specific VPS33B conditional knockout mice, had normal morphology yet their spreading on fibrinogen was impaired and they failed to support clot retraction. Platelet aggregation and ATP secretion in response to low-dose agonists were reduced in the VPS33B knockout mice. αIIbβ3-mediated endocytosis of fibrinogen was also defective. Tail bleeding times and times to occlusion in a FeCl3-induced thrombosis model were prolonged in the VPS33B knockout mice. Furthermore, VPS33B acted upstream of the RhoA-ROCK-MLC and Rac1 dependent pathways that lead to clot retraction and cell spreading, respectively.
Conclusions—Our work demonstrates that vesicular trafficking complexes, containing VPS33B, are a novel class of modifiers of integrin function. Our data also provide insights into the molecular mechanism and treatment of ARC syndrome.
- Received July 8, 2015.
- Revision received August 11, 2015.
- Accepted September 8, 2015.