title: Homers regulate calcium entry and aggregation in human platelets: a role for Homers in the association between STIM1 and Orai1.
abstract: Homer is a family of cytoplasmic adaptor proteins that play different roles in cell function, including the regulation of G-protein-coupled receptors. These proteins contain an Ena (Enabled)/VASP (vasodilator-stimulated phosphoprotein) homology 1 domain that binds to the PPXXF sequence motif, which is present in different Ca²⁺-handling proteins such as IP3 (inositol 1,4,5-trisphosphate) receptors and TRPC (transient receptor potential canonical) channels. In the present study we show evidence for a role of Homer proteins in the STIM1 (stromal interaction molecule 1)-Orai1 association, as well as in the TRPC1-IP3RII (type II IP3 receptor) interaction, which might be of relevance in platelet function. Treatment of human platelets with thapsigargin or thrombin results in a Ca²⁺-independent association of Homer1 with TRPC1 and IP3RII. In addition, thapsigargin and thrombin enhanced the association of Homer1 with STIM1 and Orai1 in a Ca²⁺-dependent manner. Interference with Homer function by introduction of the synthetic PPKKFR peptide into cells, which emulates the proline-rich sequences of the PPXXF motif, reduced STIM1-Orai1 and TRPC1- IP3RII associations, as compared with the introduction of the inactive PPKKRR peptide. The PPKKFR peptide attenuates thrombin-evoked Ca²⁺ entry and the maintenance of thapsigargin-induced store-operated Ca²⁺ entry. Finally, the PPKKFR peptide attenuated thrombin-induced platelet aggregation. The findings of the present study support an important role for Homer proteins in thrombin-stimulated platelet function, which is likely to be mediated by the support of agonist-induced Ca²⁺ entry.
authors.affiliation: Department of Physiology, Cellular Physiology Research Group, University of Extremadura, 10003 Caceres, Spain.
authors.forename: Isaac
authors.initials: I
authors.lastname: Jardin
authors.forename: Letizia
authors.initials: L
authors.lastname: Albarrán
authors.forename: Nuria
authors.initials: N
authors.lastname: Bermejo
authors.forename: Ginés M
authors.initials: GM
authors.lastname: Salido
authors.forename: Juan A
authors.initials: JA
authors.lastname: Rosado
mesh_terms: D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D015153:Blotting, Western; D015220:Calcium Channels / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002352:Carrier Proteins / Q000378:metabolism*; D049109:Cell Proliferation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D000072233:Homer Scaffolding Proteins; D006801:Humans; D047468:Immunoprecipitation; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000378:metabolism*; D000071740:ORAI1 Protein; D010446:Peptide Fragments / Q000378:metabolism; D010974:Platelet Aggregation / Q000187:drug effects; D011485:Protein Binding; D000071737:Stromal Interaction Molecule 1; D050052:TRPC Cation Channels / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
chemical_list: D015220:Calcium Channels; D002352:Carrier Proteins; D004791:Enzyme Inhibitors; C000606614:HOMER1 protein, human; D000072233:Homer Scaffolding Proteins; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D000071740:ORAI1 Protein; C510746:ORAI1 protein, human; D010446:Peptide Fragments; C103530:STIM1 protein, human; D000071737:Stromal Interaction Molecule 1; D050052:TRPC Cation Channels; C096351:transient receptor potential cation channel, subfamily C, member 1; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate