Homers regulate calcium entry and aggregation in human platelets: a role for Homers in the association between STIM1 and Orai1

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 Ca 2+ -handling proteins such as IP 3 (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–IP 3 RII (type II IP 3 receptor) interaction, which might be of relevance in platelet function. Treatment of human platelets with thapsigargin or thrombin results in a Ca 2+ -independent association of Homer1 with TRPC1 and IP 3 RII. In addition, thapsigargin and thrombin enhanced the association of Homer1 with STIM1 and Orai1 in a Ca 2+ -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– IP 3 RII associations, as compared with the introduction of the inactive PPKKRR peptide. The PPKKFR peptide attenuates thrombin-evoked Ca 2+ entry and the maintenance of thapsigargin-induced store-operated Ca 2+ 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 Ca 2+ entry.