title: The PAK system links Rho GTPase signaling to thrombin-mediated platelet activation.
abstract: Regulation of the platelet actin cytoskeleton by the Rho family of small GTPases is essential for the proper maintenance of hemostasis. However, little is known about how intracellular platelet activation from Rho GTPase family members, including Rac, Cdc42, and Rho, translate into changes in platelet actin structures. To better understand how Rho family GTPases coordinate platelet activation, we identified platelet proteins associated with Rac1, a Rho GTPase family member, and actin regulatory protein essential for platelet hemostatic function. Mass spectrometry analysis revealed that upon platelet activation with thrombin, Rac1 associates with a set of effectors of the p21-activated kinases (PAKs), including GIT1, βPIX, and guanine nucleotide exchange factor GEFH1. Platelet activation by thrombin triggered the PAK-dependent phosphorylation of GIT1, GEFH1, and other PAK effectors, including LIMK1 and Merlin. PAK was also required for the thrombin-mediated activation of the MEK/ERK pathway, Akt, calcium signaling, and phosphatidylserine (PS) exposure. Inhibition of PAK signaling prevented thrombin-induced platelet aggregation and blocked platelet focal adhesion and lamellipodia formation in response to thrombin. Together, these results demonstrate that the PAK signaling system is a key orchestrator of platelet actin dynamics, linking Rho GTPase activation downstream of thrombin stimulation to PAK effector function, MAP kinase activation, calcium signaling, and PS exposure in platelets.
authors.affiliation: Department of Biomedical Engineering, Oregon Health & Science University, Portland, Oregon, USA.
authors.forename: Joseph E
authors.initials: JE
authors.lastname: Aslan
authors.forename: Sandra M
authors.initials: SM
authors.lastname: Baker
authors.forename: Cassandra P
authors.initials: CP
authors.lastname: Loren
authors.forename: Kristina M
authors.initials: KM
authors.lastname: Haley
authors.forename: Asako
authors.initials: A
authors.lastname: Itakura
authors.forename: Jiaqing
authors.initials: J
authors.lastname: Pang
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Greenberg
authors.forename: Larry L
authors.initials: LL
authors.lastname: David
authors.forename: Ed
authors.initials: E
authors.lastname: Manser
authors.forename: Jonathan
authors.initials: J
authors.lastname: Chernoff
authors.forename: Owen J T
authors.initials: OJ
authors.lastname: McCarty
mesh_terms: D008841:Actin Cytoskeleton / Q000187:drug effects / Q000378:metabolism; D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism; D001792:Blood Platelets / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D002448:Cell Adhesion / Q000187:drug effects; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D005786:Gene Expression Regulation; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism; D006801:Humans; D054461:Lim Kinases / Q000235:genetics / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000235:genetics / Q000378:metabolism; D025581:Neurofibromin 2 / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation; D015539:Platelet Activation / Q000187:drug effects*; D010974:Platelet Aggregation / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism; D011554:Pseudopodia / Q000187:drug effects / Q000378:metabolism; D064067:Rho Guanine Nucleotide Exchange Factors; D015398:Signal Transduction; D013917:Thrombin / Q000494:pharmacology*; D054462:p21-Activated Kinases / Q000235:genetics / Q000378:metabolism*; D020830:rac1 GTP-Binding Protein / Q000235:genetics / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D018797:Cell Cycle Proteins; C485647:GIT1 protein, human; D020662:Guanine Nucleotide Exchange Factors; D025581:Neurofibromin 2; C107953:RAC1 protein, human; D064067:Rho Guanine Nucleotide Exchange Factors; C515892:LIMK1 protein, human; D054461:Lim Kinases; D051057:Proto-Oncogene Proteins c-akt; D054462:p21-Activated Kinases; D020928:Mitogen-Activated Protein Kinases; D013917:Thrombin; D020830:rac1 GTP-Binding Protein
keywords: PAK; Rac1; Rho GTPases; actin; platelets