title: Identification of an antithrombotic allosteric modulator that acts through helix 8 of PAR1.
abstract: G protein-coupled receptors (GPCRs) can assume multiple conformations and possess multiple binding sites. Whereas endogenous agonists acting at the orthosteric binding site stabilize the active receptor conformation, small molecules that act at nonorthosteric sites can stabilize alternative conformations. The large majority of these allosteric modulators associate with extracellular loops of GPCRs. The role of intracellular domains in mediating allosteric modulation is largely unknown. In screening a small-molecule library for inhibitors of platelet activation, we identified a family of compounds that modified PAR1-mediated granule secretion. The most potent inhibitory compound, termed JF5, also demonstrated noncompetitive inhibition of the α(2A)-adrenergic receptor. Aggregation studies using a battery of platelet GPCR agonists demonstrated that sensitivity to JF5 was limited to GPCRs that possessed a constrained eighth helix, as defined by a C-terminal palmitoylation site and interactions with TM7 and the i1 loop. Inhibition by JF5 was overcome in a PAR1 mutant in which the eighth helix was deleted, confirming a role for helix 8 in JF5 activity. Evaluation of downstream signaling showed that JF5 was selective with regard to G protein coupling, blocking signaling mediated by G(αq) but not G(α12). The compound inhibited thrombus formation in vivo following vascular injury with an IC(50) of ∼1 mg/kg. These results indicate a role for helix 8 in conferring sensitivity to small molecules, and show that this sensitivity can be exploited to control platelet activation during thrombus formation.
authors.affiliation: Division of Hemostasis and Thrombosis, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02115, USA.
authors.forename: Louisa
authors.initials: L
authors.lastname: Dowal
authors.forename: Derek S
authors.initials: DS
authors.lastname: Sim
authors.forename: James R
authors.initials: JR
authors.lastname: Dilks
authors.forename: Price
authors.initials: P
authors.lastname: Blair
authors.forename: Sarah
authors.initials: S
authors.lastname: Beaudry
authors.forename: Bradley M
authors.initials: BM
authors.lastname: Denker
authors.forename: Georgios
authors.initials: G
authors.lastname: Koukos
authors.forename: Athan
authors.initials: A
authors.lastname: Kuliopulos
authors.forename: Robert
authors.initials: R
authors.lastname: Flaumenhaft
mesh_terms: D000494:Allosteric Regulation / Q000502:physiology; D000818:Animals; D000991:Antithrombins / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D004285:Dogs; D004837:Epinephrine; D005434:Flow Cytometry; D008156:Luciferases; D019007:P-Selectin / Q000378:metabolism; D010446:Peptide Fragments / Q000378:metabolism; D010974:Platelet Aggregation; D017433:Protein Structure, Secondary / Q000502:physiology; D044463:Receptor, PAR-1 / Q000819:agonists / Q000378:metabolism*; D043562:Receptors, G-Protein-Coupled / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*; D013927:Thrombosis / Q000378:metabolism*
chemical_list: D000991:Antithrombins; D019007:P-Selectin; D010446:Peptide Fragments; D044463:Receptor, PAR-1; D043562:Receptors, G-Protein-Coupled; C082835:thrombin receptor peptide (42-47); D008156:Luciferases; D002118:Calcium; D004837:Epinephrine