β-Arrestin Recruitment and G Protein Signaling by the Atypical Human Chemokine Decoy Receptor CCX-CKR

Abstract

Chemokine receptors form a large subfamily of G protein-coupled receptors that predominantly activate heterotrimeric G i proteins and are involved in immune cell migration. CCX-CKR is an atypical chemokine receptor with high affinity for CCL19, CCL21, and CCL25 chemokines, but is not known to activate intracellular signaling pathways. However, CCX-CKR acts as decoy receptor and efficiently internalizes these chemokines, thereby preventing their interaction with other chemokine receptors, like CCR7 and CCR9. Internalization of fluorescently labeled CCL19 correlated with β-arrestin2-GFP translocation. Moreover, recruitment of β-arrestins to CCX-CKR in response to CCL19, CCL21, and CCL25 was demonstrated using enzyme-fragment complementation and bioluminescence resonance energy transfer methods. To unravel why CCX-CKR is unable to activate G i signaling, CCX-CKR chimeras were constructed by substituting its intracellular loops with the corresponding CCR7 or CCR9 domains. The signaling properties of chimeric CCX-CKR receptors were characterized using a cAMP-responsive element (CRE)-driven reporter gene assay. Unexpectedly, wild type CCX-CKR and a subset of the chimeras induced an increase in CRE activity in response to CCL19, CCL21, and CCL25 in the presence of the G i inhibitor pertussis toxin. CCX-CKR signaling to CRE required an intact DRY motif. These data suggest that inactive G i proteins impair CCX-CKR signaling most likely by hindering the interaction of this receptor with pertussis toxin-insensitive G proteins that transduce signaling to CRE. On the other hand, recruitment of the putative signaling scaffold β-arrestin to CCX-CKR in response to chemokines might allow activation of yet to be identified signal transduction pathways. Background: CCX-CKR is considered to be a chemokine decoy receptor that is unable to signal. Results: Chemokines induce β-arrestin recruitment to CCX-CKR and pertussis toxin (PTX)-dependent CRE activity. Conclusion: PTX-sensitive G proteins hinder CCX-CKR coupling to other G proteins and consequently keep receptors silent. Significance: Recruitment of β-arrestin to CCX-CKR requests re-evaluation of the signaling capacity of this atypical receptor.