Gα Protein Signaling Bias at Serotonin 1A Receptor

Serotonin 1A receptor (5-HT1AR) is a clinically relevant target because of its involvement in several central and peripheral functions including sleep, temperature homeostasis, processing of emotions, and response to stress. As a G Protein Coupled Receptor (GPCR) activating numerous Gai/o/z family members, 5-HT1AR can potentially modulate multiple intracellular signaling pathways in response to different therapeutics. Here, we applied a cell-based BRET assay to quantify how ten structurally diverse 5-HT1AR agonists exert biased signaling by differentially stimulating Gai/o/z family members. Our concentration-response analysis of the activation of each Gai/o/z protein revealed unique potency and efficacy profiles of selected agonists when compared to the reference 5-HT. Overall, our analysis of signaling bias identified groups of ligands sharing comparable G protein activation selectivity and also drugs with unique selectivity profiles. We observed, for example, a strong bias of F-15599 toward the activation of Gαi3 that was unique among the agonists tested: we found a biased factor of +2.19 when comparing the activation of Gαi3 versus Gαi2 by F-15599, while it was -0.29 for 8-OH-DPAT. Similarly, vortioxetine showed a biased factor of +1.06 for Gαz versus GαoA, while it was -1.38 for vilazodone. Considering that alternative signaling pathways are regulated downstream of each Ga protein, our data suggest that the unique pharmacological properties of the tested agonists could result in multiple unrelated cellular outcomes. Further investigation is needed to reveal how this type of ligand bias could affect cellular responses and to illuminate molecular mechanisms underlying therapeutic profile and side effects of each drug. Significance Statement Serotonin 1a receptor (5-HT1AR) activates several members of the Gi/o/z protein family. Here, we examined ten structurally diverse and clinically relevant agonists acting on 5-HT1AR and identified distinctive bias patterns among G proteins. Considering the diversity of their intracellular effectors and signaling properties, this data reveal novel mechanisms underlying both therapeutic and undesirable effects.