Inflammation and taste disorders: mechanisms in taste buds

Abstract

Taste disorders, including taste distortion and taste loss, negatively impact general health and quality of life. To understand the underlying molecular and cellular mechanisms, we set out to identify inflammation-related molecules in taste tissue and to assess their role in the development of taste dysfunctions. We found that 10 out of 12 mammalian Toll-like receptors (TLRs), type I and II interferon (IFN) receptors, and their downstream signaling components are present in taste tissue. Some TLRs appear to be selectively or more abundantly expressed in taste buds than in nongustatory lingual epithelium. Immunohistochemistry with antibodies against TLRs 1, 2, 3, 4, 6, and 7 confirmed the presence of these receptor proteins in taste bud cells, of which TLRs 2, 3, and 4 are expressed in the gustducin-expressing type II taste bud cells. Administration of TLR ligands, lipopolysaccharide, and double-stranded RNA polyinosinic:polycytidylic acid, which mimics bacterial or viral infection, activates the IFN signaling pathways, upregulates the expression of IFN-inducible genes, and downregulates the expression of c-fos in taste buds. Finally, systemic administration of IFNs augments apoptosis of taste bud cells in mice. Taken together, these data suggest that TLR and IFN pathways function collaboratively in recognizing pathogens and mediating inflammatory responses in taste tissue. This process, however, may interfere with normal taste transduction and taste bud cell turnover and contributes to the development of taste disorders.

Figure 1

Multiple TLR transcripts are expressed in taste epithelium. Shown here are the gel images of RT-PCR products for 12 currently known mouse TLRs as well as the positive control β-actin from nontaste lingual epithelium (NT) or from lingual epithelium containing circumvallate and foliate taste buds (CV+F). Transcripts for all TLRs except TLRs 8 and 11 were successfully amplified. M: 1 kb DNA ladder.

Figure 2

TLR receptor proteins are localized to taste bud cells. Top two rows are the confocal images of immunostaining of taste tissue sections with antibodies against TLR1 (Santa Cruz Biotechnology, SC-30000, raised against the antigenic peptide of amino acid residues 161–250 of human TLR1), TLR2 (Cell Signaling Technology, #2229, raised against the antigenic peptide of amino acid residues 179–203 of human TLR2), TLR3 (Cell Signaling Technology, #2253, raised against the antigenic peptide of amino acid residues 883-904 of human TLR3), TLR4 (eBioscience, #24–9048, raised against bacterially expressed recombinant human TLR4), TLR6 (Santa Cruz Biotechnology, SC-5662, raised against an 18-amino acid peptide between residues 75–125 of mouse TLR6) and TLR7 (eBioscience, #14–9079, raised against a 14-amino acid peptide between residues 400–450 of mouse TLR7). The antibodies raised against human TLR1-4 cross-react with the corresponding mouse TLR. Cy3-conjugated secondary antibodies were used except for TLR6 antibody, which was paired with FITC-conjugated secondary antibody. Bottom panels show the controls with primary antibody omitted or preincubated with antigenic peptides. Images were taken using a Leica (Nussloch, Germany) TCS SP2 spectral confocal microscope, which were then arranged and adjusted for contrast and brightness by using Photoshop v8 (Adobe Systems, San Jose, CA).

Figure 3

Colocalization of TLRs 2, 3, and 4 with gustducin. Circumvallate sections from gustducin-GFP transgenic mice were immunostained with the same antibodies against TLRs 2, 3 and 4 as those described in Figure 2 legend. Cells positive for TLRs 2, 3, and 4 are shown in red (left panels). Gustducin-positive cells are represented by GFP (middle panels). Merged images show the colocalization of TLRs with gustducin (right panels).