title: Modulating platelet reactivity through control of RGS18 availability.
abstract: Most platelet agonists activate platelets by binding to G-protein-coupled receptors. We have shown previously that a critical node in the G-protein signaling network in platelets is formed by a scaffold protein, spinophilin (SPL), the tyrosine phosphatase, Src homology region 2 domain-containing phosphatase-1 (SHP-1), and the regulator of G-protein signaling family member, RGS18. Here, we asked whether SPL and other RGS18 binding proteins such as 14-3-3γ regulate platelet reactivity by sequestering RGS18 and, if so, how this is accomplished. The results show that, in resting platelets, free RGS18 levels are relatively low, increasing when platelets are activated by thrombin. Free RGS18 levels also rise when platelets are rendered resistant to activation by exposure to prostaglandin I2 (PGI2) or forskolin, both of which increase platelet cyclic adenosine monophosphate (cAMP) levels. However, the mechanism for raising free RGS18 is different in these 2 settings. Whereas thrombin activates SHP-1 and causes dephosphorylation of SPL tyrosine residues, PGI2 and forskolin cause phosphorylation of SPL Ser94 without reducing tyrosine phosphorylation. Substituting alanine for Ser94 blocks cAMP-induced dissociation of the SPL/RGS/SHP-1 complex. Replacing Ser94 with aspartate prevents formation of the complex and produces a loss-of-function phenotype when expressed in mouse platelets. Together with the defect in platelet function we previously observed in SPL(-/-) mice, these data show that (1) regulated sequestration and release of RGS18 by intracellular binding proteins provides a mechanism for coordinating activating and inhibitory signaling networks in platelets, and (2) differential phosphorylation of SPL tyrosine and serine residues provides a key to understanding both.
authors.affiliation: Departments of Medicine and Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA; and.
authors.forename: Peisong
authors.initials: P
authors.lastname: Ma
authors.affiliation: Departments of Medicine and Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA; and.
authors.forename: Kristy
authors.initials: K
authors.lastname: Ou
authors.affiliation: Departments of Medicine and Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA; and.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Sinnamon
authors.affiliation: Departments of Medicine and Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA; and.
authors.forename: Hong
authors.initials: H
authors.lastname: Jiang
authors.affiliation: Department of Physiology and Pharmacology, West Virginia University School of Medicine, Morgantown, WV.
authors.forename: David P
authors.initials: DP
authors.lastname: Siderovski
authors.affiliation: Departments of Medicine and Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA; and.
authors.forename: Lawrence F
authors.initials: LF
authors.lastname: Brass
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000187:drug effects; D016466:CHO Cells; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D003412:Cricetulus; D000242:Cyclic AMP / Q000502:physiology; D011464:Epoprostenol / Q000494:pharmacology; D016332:Fetal Tissue Transplantation; D008099:Liver / Q000196:embryology; D016031:Liver Transplantation; D051379:Mice; D008840:Microfilament Proteins / Q000172:deficiency / Q000502:physiology; D009419:Nerve Tissue Proteins / Q000172:deficiency / Q000502:physiology; D010766:Phosphorylation; D010768:Phosphoserine / Q000378:metabolism; D019000:Phosphotyrosine / Q000378:metabolism; D015539:Platelet Activation / Q000502:physiology*; D025941:Protein Interaction Mapping; D011499:Protein Processing, Post-Translational; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6 / Q000502:physiology; D020710:RGS Proteins / Q000502:physiology*; D011828:Radiation Chimera; D018179:Receptors, Thrombin / Q000819:agonists; D015290:Second Messenger Systems / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology; D013917:Thrombin / Q000494:pharmacology; D000099273:Neurabins
chemical_list: D005576:Colforsin; D000242:Cyclic AMP; D011464:Epoprostenol; D008840:Microfilament Proteins; D009419:Nerve Tissue Proteins; D010768:Phosphoserine; D019000:Phosphotyrosine; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6; D020710:RGS Proteins; D018179:Receptors, Thrombin; D013917:Thrombin; D000099273:Neurabins; C422991:Rgs18 protein, mouse; C509248:Ptpn6 protein, mouse