Motile cilia of human airway epithelia are chemosensory

Abstract

Cilia are microscopic projections that extend from eukaryotic cells. There are two general types of cilia; primary cilia serve as sensory organelles, whereas motile cilia exert mechanical force. The motile cilia emerging from human airway epithelial cells propel harmful inhaled material out of the lung. We found that these cells express sensory bitter taste receptors, which localized on motile cilia. Bitter compounds increased the intracellular calcium ion concentration and stimulated ciliary beat frequency. Thus, airway epithelia contain a cell-autonomous system in which motile cilia both sense noxious substances entering airways and initiate a defensive mechanical mechanism to eliminate the offending compound. Hence, like primary cilia, classical motile cilia also contain sensors to detect the external environment.

Figure 1

Bitter taste receptors are expressed in human airway epithelia. (A) Microarray analysis of mRNA isolated from airway epithelial samples isolated from eight human donors (15). Data are mean±SEM. (B) RT-PCR analysis of mRNA isolated from differentiated cultures of human airway epithelia.

Figure 2

T2Rs localize along the motile cilia of airway epithelia. Confocal immunofluorescence microscopy of cultured human airway epithelia with anti-T2R antibodies showed that (A) T2R4 and (B) T2R43 (both green) localize to motile cilia, which were identified by antibody to acetylated α-tubulin (red). Nuclei stained with DAPI are blue. Arrows in panel B indicate location of ciliated cells that were not labeled by anti-T2R43 antibody. (C) T2R38 (red) and T2R46 (green) localize to motile cilia. Data are stacks of confocal z-series images in X-Y plane and a single X-Z plane image (right, dashed line indicates the filter). Panel B (right) also shows anti-pericentrin antibody (purple), which labels the basal body below cilia. Scale bars indicate 20 μM. See also Fig. S1–S7.

Figure 3

Components of the bitter taste transduction pathway localize to ciliated cells and to the cilia. Immunolocalization of (A) α-gustducin, (B) PLC-β2, and (C) TRPM5 are in green and acetylated α-tubulin is in red. Other aspects of Figure are as described in legend of Fig. 2. Scale bars indicate 20 μM. See also Fig. S1–S3.

Figure 4

Bitter compounds increase [Ca²⁺]_i in ciliated cells. (A) Differentiated airway epithelia were loaded with Fura2-AM and fluorescence imaging was performed to assess changes in [Ca²⁺]_i, indicated as the ratio of 340nm/380nm. Denatonium, thujone, salicin, quinine, or nicotine was added as indicated. “PBS” indicates addition of phosphate buffered saline and arrows indicate the time of addition. Data points are average of 20 cells. (B) Latency to increase in [Ca²⁺]_i following addition of 1 mM denatonium or 1 mM thujone; latency includes the time for the compound to diffuse to the cell following its addition. Asterisks indicate P<0.001, N = 20. See also Fig. S8. (C) Denatonium increases ciliary beat frequency. Data are ciliary beat frequency in differentiated human airway epithelia treated with PBS or denatonium. N = 4. Asterisks indicate P<0.01. See also Fig. S8, S9.