Aleksandra Stojanovic-Terpo

A directional switch of integrin signalling and a new anti-thrombotic strategy

Integrins have a critical role in thrombosis and haemostasis. Antagonists of the platelet integrin αIIbβ3 are potent anti-thrombotic drugs, but also have the life-threatening adverse effect of causing bleeding. It is therefore desirable to develop new antagonists that do not cause bleeding. Integrins transmit signals bidirectionally. Inside-out signalling activates integrins through a talin-dependent mechanism. Integrin ligation mediates thrombus formation and outside-in signalling, which requires Gα13 and greatly expands thrombi. Here we show that Gα13 and talin bind to mutually exclusive but distinct sites within the integrin β3 cytoplasmic domain in opposing waves. The first talin-binding wave mediates inside-out signalling and also ligand-induced integrin activation, but is not required for outside-in signalling. Integrin ligation induces transient talin dissociation and Gα13 binding to an EXE motif (in which X denotes any residue), which selectively mediates outside-in signalling and platelet spreading. The second talin-binding wave is associated with clot retraction. An EXE-motif-based inhibitor of Gα13–integrin interaction selectively abolishes outside-in signalling without affecting integrin ligation, and suppresses occlusive arterial thrombosis without affecting bleeding time. Thus, we have discovered a new mechanism for the directional switch of integrin signalling and, on the basis of this mechanism, designed a potent new anti-thrombotic drug that does not cause bleeding.

An important role for Akt3 in platelet activation and thrombosis

The Akt family of serine/threonine kinases includes Akt1, Akt2, and Akt3 isoforms. Prior studies have reported that Akt1 and Akt2, but not Akt3, are expressed in platelets. Here, we show that Akt3 is expressed in substantial amounts in platelets. Akt3(-/-) mouse platelets selectively exhibit impaired platelet aggregation and secretion in response to low concentrations of thrombin receptor agonists and thromboxane A₂ (TXA₂), but not collagen or VWF. In contrast, platelets from Akt1(-/-) or Akt2(-/-) mice are defective in platelet activation induced by thrombin, TXA₂, and VWF, but only Akt1(-/-) platelets show significant defects in response to collagen, indicating differences among Akt isoforms. Akt3(-/-) platelets exhibit a significant reduction in thrombin-induced phosphorylation of glycogen synthase kinase 3β (GSK-3β) at Ser9, which is known to inhibit GSK-3β function. Thus, Akt3 is important in inhibiting GSK-3β. Accordingly, treatment of Akt3(-/-) platelets with a GSK-3β inhibitor rescued the defect of Akt3(-/-) platelets in thrombin-induced aggregation, suggesting that negatively regulating GSK-3β may be a mechanism by which Akt3 promotes platelet activation. Importantly, Akt3(-/-) mice showed retardation in FeCl₃-induced carotid artery thrombosis in vivo. Thus, Akt3 plays an important and distinct role in platelet activation and in thrombosis.

Differential Roles of the NADPH-Oxidase 1 and 2 in Platelet Activation and Thrombosis

Objective—Reactive oxygen species (ROS) are known to regulate platelet activation; however, the mechanisms of ROS production during platelet activation remain unclear. Platelets express different isoforms of nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) oxidases (NOXs). Here, we investigated the role of NOX1 and NOX2 in ROS generation and platelet activation using NOX1 and NOX2 knockout mice. Approach and Results—NOX1−/Y platelets showed selective defects in G-protein–coupled receptor–mediated platelet activation induced by thrombin and thromboxane A2 analog U46619, but were not affected in platelet activation induced by collagen-related peptide, a glycoprotein VI agonist. In contrast, NOX2−/− platelets showed potent inhibition of collagen-related peptide-induced platelet activation, and also showed partial inhibition of thrombin-induced platelet activation. Consistently, production of ROS was inhibited in NOX1−/Y platelets stimulated with thrombin, but not collagen-related peptide, whereas NOX2−/− platelets showed reduced ROS generation induced by collagen-related peptide or thrombin. Reduced ROS generation in NOX1/2-deficient platelets is associated with impaired activation of Syk and phospholipase C&ggr;2, but minimally affected mitogen-activated protein kinase pathways. Interestingly, laser-induced arterial thrombosis was impaired but the bleeding time was not affected in NOX2−/− mice. Wild-type thrombocytopenic mice injected with NOX2−/− platelets also showed defective arterial thrombosis, suggesting an important role for platelet NOX2 in thrombosis in vivo but not hemostasis. Conclusions—NOX1 and NOX2 play differential roles in different platelet activation pathways and in thrombosis. ROS generated by these enzymes promotes platelet activation via the Syk/phospholipase C&ggr;2/calcium signaling pathway.

Role for Platelet Glycoprotein Ib-IX and Effects of its Inhibition in Endotoxemia-Induced Thrombosis, Thrombocytopenia, and Mortality

Objective—Poor prognosis of sepsis is associated with bacterial lipopolysaccharide (LPS)-induced intravascular inflammation, microvascular thrombosis, thrombocytopenia, and disseminated intravascular coagulation. Platelets are critical for thrombosis, and there has been increasing evidence of the importance of platelets in endotoxemia. The platelet adhesion receptor, the glycoprotein Ib-IX complex (GPIb-IX), mediates platelet adhesion to inflammatory vascular endothelium and exposed subendothelium. Thus, we have investigated the role of GPIb-IX in LPS-induced platelet adhesion, thrombosis, and thrombocytopenia. Approach and Results—LPS-induced mortality is significantly decreased in mice expressing a functionally deficient mutant of GPIb&agr;. Furthermore, we have developed a micellar peptide inhibitor, MP&agr;C (C13H27CONH-SIRYSGHpSL), which selectively inhibits the von Willebrand factor -binding function of GPIb-IX and GPIb-IX–mediated platelet adhesion under flow without affecting GPIb-IX–independent platelet activation. MP&agr;C inhibits platelet adhesion to LPS-stimulated endothelial cells in vitro and alleviates LPS-induced thrombosis in glomeruli in mice. Importantly, MP&agr;C reduces mortality in LPS-challenged mice, suggesting a protective effect of this inhibitor during endotoxemia. Interestingly, MP&agr;C, but not the integrin antagonist, Integrilin, alleviated LPS-induced thrombocytopenia. Conclusions—These data indicate an important role for the platelet adhesion receptor GPIb-IX in LPS-induced thrombosis and thrombocytopenia, and suggest the potential of targeting GPIb as an antiplatelet strategy in managing endotoxemia.

Signaling-mediated cooperativity between glycoprotein Ib-IX and protease-Activated receptors in thrombin-induced platelet activation

Thrombin-induced cellular response in platelets not only requires protease-activated receptors (PARs), but also involves another thrombin receptor, the glycoprotein Ib-IX complex (GPIb-IX). It remains controversial how thrombin binding to GPIb-IX stimulates platelet responses. It was proposed that GPIb-IX serves as a dock that facilitates thrombin cleavage of protease-activated receptors, but there are also reports suggesting that thrombin binding to GPIb-IX induces platelet activation independent of PARs. Here we show that GPIb is neither a passive thrombin dock nor a PAR-independent signaling receptor. We demonstrate a novel signaling-mediated cooperativity between PARs and GPIb-IX. Low-dose thrombin-induced PAR-dependent cell responses require the cooperativity of GPIb-IX signaling, and conversely, thrombin-induced GPIb-IX signaling requires cooperativity of PARs. This mutually dependent cooperativity requires a GPIb-IX-specific 14-3-3-Rac1-LIMK1 signaling pathway, and activation of this pathway also requires PAR signaling. The cooperativity between GPIb-IX signaling and PAR signaling thus drives platelet activation at low concentrations of thrombin, which are important for in vivo thrombosis.

Agonist-induced platelet procoagulant activity requires shear and a Rac1-dependent signaling mechanism

Activated platelets facilitate blood coagulation by exposing phosphatidylserine (PS) and releasing microvesicles (MVs). However, the potent physiological agonists thrombin and collagen poorly induce PS exposure when a single agonist is used. To obtain a greater procoagulant response, thrombin is commonly used in combination with glycoprotein VI agonists. However, even under these conditions, only a percentage of platelets express procoagulant activity. To date, it remains unclear why platelets poorly expose PS even when stimulated with multiple agonists and what the signaling pathways are of soluble agonist-induced platelet procoagulant activity. Here we show that physiological levels of shear present in blood significantly enhance agonist-induced platelet PS exposure and MV release, enabling low doses of a single agonist to induce full-scale platelet procoagulant activity. PS exposed on the platelet surface was immediately released as MVs, revealing a tight coupling between the 2 processes under shear. Using platelet-specific Rac1(-/-) mice, we discovered that Rac1 plays a common role in mediating the low-dose agonist-induced procoagulant response independent of platelet aggregation, secretion, and the apoptosis pathway. Platelet-specific Rac1 function was not only important for coagulation in vitro but also for fibrin accumulation in vivo following laser-induced arteriolar injury.

LIM kinase-1 selectively promotes glycoprotein Ib-IX-mediated TXA2 synthesis, platelet activation, and thrombosis

Current antithrombotic drugs have an adverse effect on bleeding, highlighting the need for new molecular targets for developing antithrombotic drugs that minimally affect hemostasis. Here we show that LIMK1(-/-) mice have defective arterial thrombosis in vivo but do not differ from wild-type mice with respect to bleeding time. LIMK1(-/-) mice show a selective defect in platelet activation induced through the von Willebrand Factor (VWF) receptor, the glycoprotein Ib-IX-V complex (GPIb-IX), but not by GPIb-IX-independent platelet agonists. In fact, LIMK1(-/-) platelets show an enhanced reaction to certain GPIb-IX-independent agonists. The defect of LIMK1(-/-) platelets in GPIb-IX-mediated platelet activation is attributed to a selective inhibition in VWF/GPIb-IX-induced phosphorylation of cytosolic phospholipase A2 (cPLA2) and consequent thromboxane A2 (TXA2) production. Supplementing a TXA2 analog, U46619, corrected the defect of LIMK1(-/-) platelets in VWF-induced stable platelet adhesion. Although LIMK1(-/-) platelets also showed reduced actin polymerization after GPIb-IX-mediated platelet aggregation, actin polymerization inhibitors did not reduce TXA2 generation, but rather accelerated platelet aggregation, suggesting that the role of LIMK1 in GPIb-mediated platelet activation is independent of actin polymerization. Thus, LIMK1 plays a novel role in selectively mediating GPIb-IX-dependent TXA2 synthesis and thrombosis and represents a potential target for developing antithrombotic drugs with minimal bleeding side effect.

G protein-dependent basal and evoked endothelial cell vWF secretion

von Willebrand factor (vWF) secretion by endothelial cells (ECs) is essential for hemostasis and thrombosis; however, the molecular mechanisms are poorly understood. Interestingly, we observed increased bleeding in EC-Gα13(-/-);Gα12(-/-) mice that could be normalized by infusion of human vWF. Blood from Gα12(-/-) mice exhibited significantly reduced vWF levels but normal vWF multimers and impaired laser-induced thrombus formation, indicating that Gα12 plays a prominent role in EC vWF secretion required for hemostasis and thrombosis. In isolated buffer-perfused mouse lungs, basal vWF levels were significantly reduced in Gα12(-/-), whereas thrombin-induced vWF secretion was defective in both EC-Gαq(-/-);Gα11(-/-) and Gα12(-/-) mice. Using siRNA in cultured human umbilical vein ECs and human pulmonary artery ECs, depletion of Gα12 and soluble N-ethylmaleimide-sensitive-fusion factor attachment protein α (α-SNAP), but not Gα13, inhibited both basal and thrombin-induced vWF secretion, whereas overexpression of activated Gα12 promoted vWF secretion. In Gαq, p115 RhoGEF, and RhoA-depleted human umbilical vein ECs, thrombin-induced vWF secretion was reduced by 40%, whereas basal secretion was unchanged. Finally, in vitro binding assays revealed that Gα12 N-terminal residues 10-15 mediated the binding of Gα12 to α-SNAP, and an engineered α-SNAP binding-domain minigene peptide blocked basal and evoked vWF secretion. Discovery of obligatory and complementary roles of Gα12 and Gαq/11 in basal vs evoked EC vWF secretion may provide promising new therapeutic strategies for treatment of thrombotic disease.

Shear-induced integrin signaling in platelet phosphatidylserine exposure, microvesicle release, and coagulation

It is currently unclear why agonist-stimulated platelets require shear force to efficiently externalize the procoagulant phospholipid phosphatidylserine (PS) and release PS-exposed microvesicles (MVs). We reveal that integrin outside-in signaling is an important mechanism for this requirement. PS exposure and MV release were inhibited in β3-/- platelets or by integrin antagonists. The impaired MV release and PS exposure in β3-/- platelets were rescued by expression of wild-type β3 but not a Gα13 binding-deficient β3 mutant (E733EE to AAA), which blocks outside-in signaling but not ligand binding. Inhibition of Gα13 or Src also diminished agonist/shear-dependent PS exposure and MV release, further indicating a role for integrin outside-in signaling. PS exposure in activated platelets was induced by application of pulling force via an integrin ligand, which was abolished by inhibiting Gα13-integrin interaction, suggesting that Gα13-dependent transmission of mechanical signals by integrins induces PS exposure. Inhibition of Gα13 delayed coagulation in vitro. Furthermore, inhibition or platelet-specific knockout of Gα13 diminished laser-induced intravascular fibrin formation in arterioles in vivo. Thus, β3 integrins serve as a shear sensor activating the Gα13-dependent outside-in signaling pathway to facilitate platelet procoagulant function. Pharmacological targeting of Gα13-integrin interaction prevents occlusive thrombosis in vivo by inhibiting both coagulation and platelet thrombus formation.