The platelet fibrinogen receptor, integrin ␣ IIb ␤ 3 , is a noncovalent heterodimer of glycoproteins IIb and IIIa. This work was aimed at elucidating the role played by the carboxy-terminal extracellular, transmembrane, and cytoplasmic regions of the glycoprotein ␤ 3 in the formation of functional complexes with ␣ subunits. Progressive carboxy-terminal deletions of ␤ 3 revealed that surface exposure of ␣ IIb ␤ 3 or ␣ v ␤ 3 could not occur in the absence of the transmembrane domain of ␤ 3 . In con- IntroductionThe glycoprotein (GP) IIb-IIIa complex, integrin ␣ IIb ␤ 3 , is a calcium-dependent, noncovalent heterodimer formed by GPIIb and GPIIIa. This complex is found in the plasma membrane of megakaryocytes, platelets, and some tumor tissues 1-3 and functions as a receptor for fibrinogen and other adhesive proteins like the von Willebrand factor, fibronectin, or vitronectin. 4 The ␤ 3 subunit may also complex the GP ␣ v to form the vitronectin receptor (integrin ␣ v ␤ 3 ) that shares with ␣ IIb ␤ 3 the binding of fibrinogen although with different affinity. 5 The platelet ␣ IIb ␤ 3 complex is essential to maintain a normal hemostasis. Unlike other platelet receptors that are constitutively active, the ␣ IIb ␤ 3 is maintained in a low-affinity state for its ligands. Disruption of the vascular endothelium and exposure of platelets to the action of agonists and adhesive proteins from the subendothelial matrix induces a cellular activation. The activated cells interact with adhesive proteins from the extracellular matrix, 6,7 and the ␣ IIb ␤ 3 receptors are able to bind fibrinogen with high affinity (insideout signaling), resulting in platelet aggregation. 8 Conversely, ligand-bound ␣ IIb ␤ 3 propagates signals to the interior of the cell (outside-in signaling) leading to enhanced interaction with the cytoskeleton, clustering of receptors (increased ligand avidity), and formation of focal contacts rich in signaling complexes. 9,10 The agonist-induced increase in ligand affinity of ␣ IIb ␤ 3 is thought to be the result of conformational changes of the heterodimer [11][12][13] initiated by the interaction of the cytoplasmic tails of ␣ and ␤ 3 subunits with cytosolic proteins. Despite the pathophysiologic importance of the platelet ␣ IIb ␤ 3 receptor, the knowledge of the mechanisms controlling its state of activation is rather limited.Unlike previous reports, 14 recent work from our laboratory 15 showed that a truncated form of ␤ 3 lacking the transmembrane and cytosolic domains failed to associate with ␣ IIb . The present work was aimed at further investigating the role played by the carboxyterminal domain of ␤ 3 in the surface expression and function of ␤ 3 heterodimers. The results obtained in this study indicate that surface expression of ␣ IIb ␤ 3 could not occur in the absence of the transmembrane domain of ␤ 3 . The present study has also revealed that either deletion of the carboxy-terminal region of the ␤ 3 ectodomain or disruption of the 663-687 disulfide bridge confers constitutive activity to the ␤ 3 integr...

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Podocalyxin (Podxl) has an essential role in the development and function of the kidney glomerular filtration barrier. It is also expressed by vascular endothelia but perinatal lethality of podxl(-/-) mice has precluded understanding of its function in adult vascular endothelial cells (ECs). In this work, we show that conditional knockout mice with deletion of Podxl restricted to the vascular endothelium grow normally but most die spontaneously around three months of age. Histological analysis showed a nonspecific inflammatory infiltrate within the vessel wall frequently associated with degenerative changes, and involving vessels of different caliber in one or more organs. Podxl-deficient lung EC cultures exhibit increased permeability to dextran and macrophage transmigration. After thrombin stimulation, ECs lacking Podxl showed delayed recovery of VE-cadherin cell contacts, persistence of F-actin stress fibers, and sustained phosphorylation of the ERM complex and activation of RhoA, suggesting a failure in endothelial barrier stabilization. The results suggest that Podxl has an essential role in the regulation of endothelial permeability by influencing the mechanisms involved in the restoration of endothelial barrier integrity after injury.