Coupling of Airway Smooth Muscle Bitter Taste Receptors to Intracellular Signaling and Relaxation Is via G αi1,2,3

Bitter taste receptors (TAS2Rs) are expressed on human airway smooth muscle (HASM) and evoke marked relaxation. Agonist interaction with TAS2Rs activates phospholipase C and increases compartmentalized intracellular Ca ([Ca ] ) via inositol 1,4,5 triphosphate. In taste cells, the G protein gustducin couples TAS2R to phospholipase C; however, we find very low levels of G mRNA or protein in HASM. We hypothesized that another G protein in HASM transmits TAS2R function. TAS2R signaling to [Ca ] , extracellular signal-regulated kinase (ERK) 1/2, and physiologic relaxation was sensitive to pertussis toxin, confirming a role for a member of the G family. α subunit expression in HASM was G > G = G > G ≈ G , with G and G at the limits of detection (>100-fold lower than G ). Small interfering RNA knockdowns in HASM showed losses of [Ca ] and ERK1/2 signaling when G , G , or G were reduced. G and G knockdowns had no effect on [Ca ] and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, G and G knockdowns did not affect any TAS2R signaling. In overexpression experiments in human embryonic kidney-293T cells, we confirmed an agonist-dependent physical interaction between TAS2R14 and G . ASM cells from transgenic mice expressing a peptide inhibitor of G had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, G , G , and G , with no evidence for functional coupling to G . This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of G , indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.