OC 78.4 - GRK3 regulates platelet function through general G protein-coupled receptor desensitization

Background: Many agonists exert their cellular effects by engaging G protein-coupled receptors (GPCRs) to cause platelet activation and GPCR kinases (GRKs) regulate various GPCR-mediated signaling by binding with β-arrestins in different cells. However, the significance of GRK-arrestin complexes in the regulation of GPCR-mediated platelet functional responses has not been clearly understood.

Aims: We examined the functional significance of GRK3 and its molecular mechanisms in the regulation of GPCR signaling in platelets.

Methods: GRK3 and β-arrestin2 -/- mice were used to evaluate their functional role in platelet activation.

Results: GPCR agonists including 2-MeSADP-, AYPGKF-, thrombin-, and thromboxane A2 analog U46619-induced platelet aggregation and dense granule secretion were significantly potentiated in GRK3 -/- platelets compared to wild-type platelets. Conversely, platelet aggregation and secretion induced by the non-GPCR agonist collagen were unaffected, suggesting that GRK3 regulates GPCR-mediated platelet function. We have demonstrated earlier that β-arrestin2 plays a central role in the regulation of GPCR-mediated platelet function. Interestingly, the extent of potentiation of GPCR agonists-stimulated platelet function was less in GRK3 -/- platelets compared to β-arrestin2 -/- platelets, indicating that there may be the contribution of other GRK isoforms in the regulation of GPCR-mediated platelet functional responses. In addition, we have previously demonstrated that GRK6 is not involved in the regulation of Gq-coupled 5-HT2A and Gz-coupled α2A adrenergic receptors. Surprisingly, in contrast to GRK6, platelet aggregation and secretion induced by the co-stimulation of serotonin and epinephrine were significantly potentiated in GRK3 -/- platelets, suggesting the role of GRK3 in the general GPCR regulation. Moreover, platelet aggregation was restored following the second exposure to ADP in GRK3 -/- platelets, confirming that GRK3 is involved in GPCR desensitization in platelets. Finally, 2-MeSADP- and AYPGKF-induced AKT and ERK phosphorylation were significantly potentiated in GRK3 -/- platelets.

Conclusion(s): GRK3 plays an important role in regulating the platelet functional responses through general GPCR desensitization.