Coupling of Airway Smooth Muscle Bitter Taste Receptors to Intracellular Signaling and Relaxation Is via G?i1,2,3.

ABSTRACT: 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 Ca2+ ([Ca2+]i) 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?gust mRNA or protein in HASM. We hypothesized that another G protein in HASM transmits TAS2R function. TAS2R signaling to [Ca2+]i, extracellular signal-regulated kinase (ERK) 1/2, and physiologic relaxation was sensitive to pertussis toxin, confirming a role for a member of the Gi family. ? subunit expression in HASM was G?i2?>?G?i1?=?G?i3?>?G?trans1???G?trans2, with G?gust and G?o at the limits of detection (>100-fold lower than G?i2). Small interfering RNA knockdowns in HASM showed losses of [Ca2+]i and ERK1/2 signaling when G?i1, G?i2, or G?i3 were reduced. G?trans1 and G?trans2 knockdowns had no effect on [Ca2+]i and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, G?gust and G?o 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?i2. ASM cells from transgenic mice expressing a peptide inhibitor of G?i2 had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, G?i1, G?i2, and G?i3, with no evidence for functional coupling to G?gust. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of G?gust, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.

SUBMITTER: Kim D 

PROVIDER: S-EPMC5516295 | biostudies-literature | 2017 Jun

REPOSITORIES: biostudies-literature