Solitary chemosensory cells and bitter taste receptor signaling in human sinonasal mucosa

Abstract

Background: Solitary chemosensory cells (SCCs) are specialized cells in the respiratory epithelium that respond to noxious chemicals including bacterial signaling molecules. SCCs express components of bitter taste transduction including the taste receptor type 2 (TAS2R) bitter taste receptors and downstream signaling effectors: α-Gustducin, phospholipase Cβ2 (PLCβ2), and transient receptor potential cation channel subfamily M member 5 (TRPM5). When activated, SCCs evoke neurogenic reflexes, resulting in local inflammation. The purpose of this study was to test for the presence SCCs in human sinonasal epithelium, and to test for a correlation with inflammatory disease processes such as allergic rhinitis and chronic rhinosinusitis.

Methods: Patient demographics and biopsies of human sinonasal mucosa were obtained from control patients (n = 7) and those with allergic rhinitis and/or chronic rhinosinusitis (n = 15). Reverse transcription polymerase chain reaction (RT-PCR), quantitative PCR (qPCR), and immunohistochemistry were used to determine whether expression of signaling effectors was altered in diseased patients.

Results: RT-PCR demonstrated that bitter taste receptors TAS2R4, TAS2R14, and TAS2R46, and downstream signaling effectors α-Gustducin, PLCβ2, and TRPM5 are expressed in the inferior turbinate, middle turbinate, septum, and uncinate of both control and diseased patients. PLCβ2/TRPM5-immunoreactive SCCs were identified in the sinonasal mucosa of both control and diseased patients. qPCR showed similar expression of α-Gustducin and TRPM5 in the uncinate process of control and diseased groups, and there was no correlation between level of expression and 22-item Sino-Nasal Outcomes Test (SNOT-22) or pain scores.

Conclusion: SCCs are present in human sinonasal mucosa in functionally relevant areas. Expression level of signaling effectors was similar in control and diseased patients and did not correlate with measures of pain and inflammation. Further study into these pathways may provide insight into nasal inflammatory diseases and may offer potential therapeutic targets.

Figure 1

GelRed stained RT-PCR products from the mRNA of human nasal tissue taken from a patient with CRS amplified with primers designed for (A) PLCβ2, (B) α-Gustducin, (C) TRPM5, (D) TAS2R4, (E) TAS2R14, and (F) TAS2R46. Expected sizes for PCR products are as followed: PLCβ2: 333bp, α-Gustducin: 386bp, TRPM5: 301bp, TAS2R4: 474bp, TAS2R14: 250bp, and TAS2R46: 250bp.Lanes represent (in order): negative control (no template); positive control, human tongue (HT, Clontech); inferior turbinate (IT); middle turbinate (MT); septum (S); and uncinate (U). In general, all regions biopsied showed expression of taste signaling effectors in both diseased patients and control patients (refer to Table 1).

Figure 2

Immunohistochemistry for PLCβ2 (A-C) and TRPM5 (D-F) from human biopsy material. A. A presumed PLCβ2-immunoreactive solitary chemosensory cell from the septum of a CRS patient. B. PLCβ2-immunoreactive cells in the inferior turbinate of a control subject. C. PLCβ2-immuoreactive taste cells in a circumvallate papilla for validation of antibody. D. TRPM5-immunoreactive cell in a whole mount of the posterior part of the middle turbinate. E. Cluster of TRPM5-immunoreactive cells in the middle turbinate in sections taken from the whole mount specimen shown in panel D. F. Low magnification view of the whole mount middle turbinate showing the density of labeled cells in a region containing many presumed SCCs. Scale bar: 25 μM (Panels A-E); 50 μM for panel F.

Figure 3

Quantitative PCR for α-Gustducin and TRPM5 mRNA in the uncinate from control and diseased (CRS/AR) patients. Expression is shown relative to an internal calibrator. There was no significant difference in expression of either gene between the two groups of patients. Data are represented as means ± SEM from control (n=7) and diseased (n=8) patients, each run in triplicates.

Figure 4

Relationship between SNOT-22 score and relative expression ofα-Gustducin and TRPM5 in control (open circles) and diseased (closed circles) patients (upper panels) and VAS pain score and α-Gustducin and TRPM5 (lower panels). Linear regression analysis showed no correlation between SNOT-22 score and relative expression level of either gene (r²= 0.01 for α-Gustducin and 0.003 for TRPM5) and VAS pain score and either gene (r²= 0.004 for α-Gustducin and 0.017 for TRPM5). In order to control for confounding effects, multiple regression of SNOT-22 and VAS scores were run as independent variables. No significant correlations were observed.