Abstract
Platelet activation by thrombin or thrombin receptor-activating peptide (TRAP) results in extensive actin reorganization that leads to filopodia emission and lamellae spreading concomitantly with activation of the Rho family small G proteins, Cdc42 and Rac1. Evidence has been provided that direct binding of Cdc42-guanosine triphosphate (GTP) and Rac1-GTP to the N-terminal regulatory domain of the p21-activated kinase (PAK) stimulates PAK activation and actin reorganization. In the present study, we have investigated the relationship between shape change and PAK activation. We show that thrombin, TRAP, or monoclonal antibody (MoAb) anti-Fc(gamma)RIIA IV.3 induces an activation of Cdc42 and Rac1. The GpVI ligand, convulxin (CVX), that forces platelets to lamellae spreading efficiently activates Rac1. Thrombin, TRAP, MoAb IV.3, and CVX stimulate autophosphorylation and kinase activity of PAK. Inhibition of Cdc42 and Rac1 with clostridial toxin B inhibits PAK activation and lamellae spreading. The cortical-actin binding protein, p80/85 cortactin, is constitutively associated with PAK in resting platelets and dissociates from PAK after thrombin stimulation. Inhibition of PAK autophosphorylation by toxin B prevents the dissociation of cortactin. These results suggest that Cdc42/Rac1-dependent activation of PAK may trigger early platelet shape change, at least in part through the regulation of cortactin binding to PAK.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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Actin Cytoskeleton / metabolism
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Actin Cytoskeleton / ultrastructure
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Actins / metabolism
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Adult
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Antibodies, Monoclonal / pharmacology
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Antigens, CD / immunology
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Biopolymers
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Blood Platelets / drug effects
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Blood Platelets / enzymology
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Blood Platelets / ultrastructure*
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Cell Size
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Cortactin
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Crotalid Venoms / pharmacology
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Enzyme Activation
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Guanosine Triphosphate / physiology
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Humans
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Kinetics
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Lectins, C-Type*
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Microfilament Proteins / physiology*
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Phosphorylation / drug effects
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Platelet Activation / drug effects
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Protein Binding
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Protein Processing, Post-Translational / drug effects
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Protein Serine-Threonine Kinases / physiology*
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Protein Structure, Tertiary
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Proteins / pharmacology
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Pseudopodia / physiology*
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Receptors, IgG / immunology
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Receptors, Thrombin
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Recombinant Fusion Proteins / physiology
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Signal Transduction
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Thrombin / pharmacology
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cdc42 GTP-Binding Protein / physiology*
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p21-Activated Kinases
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rac1 GTP-Binding Protein / physiology*
Substances
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Actins
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Antibodies, Monoclonal
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Antigens, CD
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Biopolymers
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CTTN protein, human
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Cortactin
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Crotalid Venoms
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Fc gamma receptor IIA
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Lectins, C-Type
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Microfilament Proteins
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Proteins
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Receptors, IgG
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Receptors, Thrombin
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Recombinant Fusion Proteins
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convulxin
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Guanosine Triphosphate
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PAK1 protein, human
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PAK2 protein, human
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Protein Serine-Threonine Kinases
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p21-Activated Kinases
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Thrombin
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cdc42 GTP-Binding Protein
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rac1 GTP-Binding Protein