Coupling of Airway Smooth Muscle Bitter Taste Receptors to Intracellular Signaling and Relaxation Is via Gαi1,2,3

Am J Respir Cell Mol Biol. 2017 Jun;56(6):762-771. doi: 10.1165/rcmb.2016-0373OC.

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 Gαgust 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. α subunit expression in HASM was Gαi2 > Gαi1 = Gαi3 > Gαtrans1 ≈ Gαtrans2, with Gαgust and Gαo at the limits of detection (>100-fold lower than Gαi2). Small interfering RNA knockdowns in HASM showed losses of [Ca2+]i and ERK1/2 signaling when Gαi1, Gαi2, or Gαi3 were reduced. Gαtrans1 and Gαtrans2 knockdowns had no effect on [Ca2+]i and a minimal, transient effect on ERK1/2 phosphorylation. Furthermore, Gαgust and Gαo 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 Gαi2. ASM cells from transgenic mice expressing a peptide inhibitor of Gαi2 had attenuated relaxation to TAS2R agonist. These data indicate that, unlike in taste cells, TAS2Rs couple to the prevalent G proteins, Gαi1, Gαi2, and Gαi3, with no evidence for functional coupling to Gαgust. This absence of function for the "canonical" TAS2R G protein in HASM may be due to the very low expression of Gαgust, indicating that TAS2Rs can optionally couple to several G proteins in a cell type-dependent manner contingent upon G protein expression.

Keywords: G proteins; airway smooth muscle; asthma; bitter taste receptors; chronic obstructive pulmonary disease.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Amino Acid Sequence
  • Animals
  • GTP-Binding Protein alpha Subunits, Gi-Go / chemistry
  • GTP-Binding Protein alpha Subunits, Gi-Go / metabolism*
  • Gene Knockdown Techniques
  • HEK293 Cells
  • Humans
  • Intracellular Space / drug effects
  • Intracellular Space / metabolism
  • Lung / metabolism*
  • Mice, Transgenic
  • Muscle Relaxation* / drug effects
  • Muscle, Smooth / metabolism*
  • Peptides / pharmacology
  • Pertussis Toxin / toxicity
  • Protein Subunits / metabolism
  • RNA, Small Interfering / metabolism
  • Receptors, G-Protein-Coupled / metabolism*
  • Signal Transduction* / drug effects

Substances

  • Peptides
  • Protein Subunits
  • RNA, Small Interfering
  • Receptors, G-Protein-Coupled
  • taste receptors, type 2
  • Pertussis Toxin
  • GTP-Binding Protein alpha Subunits, Gi-Go