Biased Signaling at Chemokine Receptors

Background: GPCRs can activate selective signaling pathways according to the nature of the bound ligand. Results: Coupling selectivity of chemokine receptors CCR2, CCR5, and CCR7 with G protein subtypes was measured and compared with data obtained with other functional readouts. Conclusion: Some signaling bias was detected at CCR2 and CCR5. Significance: Signaling bias appears relatively subtle for natural ligands such as chemokines. The ability of G protein-coupled receptors (GPCRs) to activate selective signaling pathways according to the conformation stabilized by bound ligands (signaling bias) is a challenging concept in the GPCR field. Signaling bias has been documented for several GPCRs, including chemokine receptors. However, most of these studies examined the global signaling bias between G protein- and arrestin-dependent pathways, leaving unaddressed the potential bias between particular G protein subtypes. Here, we investigated the coupling selectivity of chemokine receptors CCR2, CCR5, and CCR7 in response to various ligands with G protein subtypes by using bioluminescence resonance energy transfer biosensors monitoring directly the activation of G proteins. We also compared data obtained with the G protein biosensors with those obtained with other functional readouts, such as β-arrestin-2 recruitment, cAMP accumulation, and calcium mobilization assays. We showed that the binding of chemokines to CCR2, CCR5, and CCR7 activated the three Gα i subtypes (Gα i1 , Gα i2 , and Gα i3 ) and the two Gα o isoforms (Gα oa and Gα ob ) with potencies that generally correlate to their binding affinities. In addition, we showed that the binding of chemokines to CCR5 and CCR2 also activated Gα 12 , but not Gα 13 . For each receptor, we showed that the relative potency of various agonist chemokines was not identical in all assays, supporting the notion that signaling bias exists at chemokine receptors.