Taste and smell GPCRs in the lung: Evidence for a previously unrecognized widespread chemosensory system

Abstract

Taste and smell receptor expression has been traditionally limited to the tongue and nose. We have identified bitter taste receptors (TAS2Rs) and olfactory receptors (ORs) on human airway smooth muscle (HASM) cells. TAS2Rs signal to PLCβ evoking an increase in [Ca²⁺]_i causing membrane hyperpolarization and marked HASM relaxation ascertained by single cell, ex vivo, and in vivo methods. The presence of TAS2Rs in the lung was unexpected, as was the bronchodilatory function which has been shown to be due to signaling within specific microdomains of the cell. Unlike β₂-adrenergic receptor-mediated bronchodilation, TAS2R function is not impaired in asthma and shows little tachyphylaxis. HASM ORs do not bronchodilate, but rather modulate cytoskeletal remodeling and hyperplasia, two cardinal features of asthma. We have shown that short chain fatty acids, byproducts of fermentation of polysaccharides by the gut microbiome, activate HASM ORs. This establishes a non-immune gut-lung mechanism that ties observations on gut microbial communities to asthma phenotypes. Subsequent studies by multiple investigators have revealed expression and specialized functions of TAS2Rs and ORs in multiple cell-types and organs throughout the body. Collectively, the data point towards a previously unrecognized chemosensory system which recognizes endogenous and exogenous agonists. These receptors and their ligands play roles in normal homeostatic functions, predisposition or adaptation to disease, and represent drug targets for novel therapeutics.

Keywords: Airway; Asthma; Bronchodilator; G-protein; Receptor; Smooth muscle.

Figure 1

TAS2R expression and function in human airway smooth muscle (HASM). (A) Relative expression of the 25 TAS2R subtypes in HASM as determined by quantitative RT-PCR. A high expressing (ADORA1) and low expressing (LTB4R) GPCR known to be expressed on HASM were controls. (B) [Ca²⁺]_i response to TAS2R agonists. Bradykinin and histamine, acting at Gq-coupled receptors, acted as positive controls. The bitter substance salicin is an agonist for TAS2R16 which is not expressed (see A), and showed no [Ca²⁺]_i response. Results are from 4–6 experiments. *p < 0.05 vs basal. (C) Relaxation of intact mouse airway by the TAS2R agonists quinine and chloroquine. Airways were precontracted with serotonin (n = 4 experiments). The inset shows [Ca^2+]_i traces in HASM in response to increasing doses of chloroquine. (D) Maximal relaxation of intact human airways to the β-agonist isoproterenol and the TAS2R agonists quinine and chloroquine. *, p < 0.05 vs isoproterenol (n = 5).

Figure 2

Single cell responses to TAS2R agonists in HASM. (A) Fourier transformed traction microscopy of a single HASM before and 30 sec after exposure to the TAS2R agonist chloroquine. The colors show the magnitudes of the various traction stresses throughout the cell in Pa. The wavelength of the color is proportional to the stress (red > blue; black = 0). (B) Magnetic twisting cytometry (MTC) studies of HASM show a relaxation response to TAS2R agonists chloroquine and saccharin. The expected relaxation to the β-agonist isoproterenol (ISO) and the contraction response to histamine (Hist) are also indicated. (C) MTC reveals physiologic responses to various pathway blockers in HASM. The relaxation response to saccharin was not affected by the PKA inhibitor H89, was partially inhibited by Ca²⁺-dependent K⁺ channel antagonists charybdotoxin (ChTx) and iberiotoxin (IbTx), and fully blocked by the PLCβ inhibitor U73122. (D) The relaxation response to quinine is preserved in asthmatic HASM. The MTC data in (B–D) are from > 500 measurements per condition.

Figure 3

Properties of OR51E2 expressed on human airway smooth muscle. (A) Cytoskeletal remodeling in HASM is inhibited by OR51E2 agonists. Shown are representative trajectory maps of unforced ferrimagnetic beads attached to HASM using SNTM methodology (see text). Acetate and propionate, but not formate or butyrate, inhibited spontaneous cytoskeletal motions of HASM. (B) EdU incorporation reveals that propionate decreases HASM proliferation. EdU-positive nuclei are labeled with Alexa Fluor 647 (purple color) while all nuclei are labeled with DAPI (blue color). Shown is a representative study. (C) HASM proliferation is inhibited by OR51E2 agonists acetate and proprionate (n = 4 experiments). *, p < 0.05 vs untreated. (D) Inhibition of HASM cell proliferation by OR51E2 agonists is preserved in cells derived from asthmatic lungs (color code same as in C).