Dynamic Pathway Selectivity of TAS2R5 toward or Away from β-Arrestin or G Protein from Biased Agonists

Bitter taste receptors (TAS2Rs) are GPCRs functionally expressed in extraoral organs including the lung, heart, brain, and gastrointestinal system and are candidate targets for novel drugs. It is unclear whether TAS2R5 can be stabilized by structurally distinct agonists resulting in pathway selectivity (biasing) to achieve optimal outcomes. We screened TAS2R5 agonists for signaling toward or away from β-arrestin or G protein ([Ca²⁺]_i). Agonists T5-1 (1,10-phenanthroline) and T5-6 (4,7-dimethyl-1,10-phenanthroline) coupled equally to G protein but showed marked differences in homologous desensitization (∼21% and ∼91%, respectively). This desensitization was due to dissimilar degrees of β-arrestin engagement compared to the reference agonist T5-7: T5-6 evoked up to ∼86% enhancement of β-arrestin1 or β-arrestin2 recruitment, while T5-1 was biased in the opposite direction, up to ∼78% less, compared to T5-7. T5-1 elicited little receptor internalization compared to T5-6 consistent with the β-arrestin findings. For the initial response, T5-1 and T5-6 represent one-pathway differences (decreased or increased β-arrestin) leading to bias in different directions, while G protein signaling was equivalent. However, under desensitizing conditions T5-6 was decoupled from G protein representing two-pathway biasing in opposing directions for T5-6 (increased β-arrestin and decreased G protein), imposing extreme biasing of β-arrestin over G protein. Our finding that agonists can affect β-arrestin biasing in either direction suggests that TAS2R5 is sufficiently pliable for specific signals to be engineered depending on the desired therapeutic outcome. The dynamic nature of the directionally contrasting changes of the two pathways over time accentuated bias and is due to the interaction of the two signals.