The taste of carbonation

Abstract

Carbonated beverages are commonly available and immensely popular, but little is known about the cellular and molecular mechanisms underlying the perception of carbonation in the mouth. In mammals, carbonation elicits both somatosensory and chemosensory responses, including activation of taste neurons. We have identified the cellular and molecular substrates for the taste of carbonation. By targeted genetic ablation and the silencing of synapses in defined populations of taste receptor cells, we demonstrated that the sour-sensing cells act as the taste sensors for carbonation, and showed that carbonic anhydrase 4, a glycosylphosphatidylinositol-anchored enzyme, functions as the principal CO2 taste sensor. Together, these studies reveal the basis of the taste of carbonation as well as the contribution of taste cells in the orosensory response to CO2.

Fig. 1. PKD2L1-expressing sour-sensing cells mediate taste responses to carbonation

(a) Wild-type mice show neural responses to carbonated solutions and carbon dioxide. Shown are chorda tympani nerve responses to control sweet (30mM acesulfameK, AceK), bitter (10mM quinine, QUI), salty (120mM NaCl), amino-acid (30mM mono potassium glutamate+0.5mM inosine mono phosphate, MPG), and sour (50mM Citric acid) stimuli as well as carbonated water (Club soda) and gaseous CO₂ normalized to the responses to 250 mM NaCl (NR; see Materials and Methods). (b–c) Dose-response to CO₂ in wild-type mice or in animals lacking sour-sensing cells (PKD2L1-DTA), and in control animals lacking sweet-sensing cells (T1R2-DTA). (c) Quantitation of carbon dioxide responses in wild-type (n=6), T1R2-DTA (n=4) and PKD2L1-DTA (n=5) animals. The values are mean ± sem of normalized chorda tympani responses.

Fig. 2. Selective localization of carbonic anhydrase 4 to PKD2L1-expressing sour cells

(a) Immunohistochemal staining of Car4 expression (lower panel, red) in taste buds of transgenic mice in which sour-sensing cells were marked by GFP fluorescence (PKD2L1-GFP; upper panel, green); large panel shows the superimposed double labeling. (b) Diphtheria toxin-mediated ablation of sour cells. Upper panel: Double label immunofluorescence with a marker of taste receptor cells, claudin 7 (Cldn7, blue), and antibodies to Car4 (red). Lower panel: labeling in PKD2L1-DTA mice stained as above. Shown are sections of foliate papillae, equivalent results were obtained in taste buds from other regions of the oral cavity (Supplementary Fig. S4).

Figure 3. Requirement of carbonic anhydrase 4 for taste responses to carbon dioxide

(a) Representative integrated chorda tympani responses to CO₂ and sour stimulation in wild type or Car4^−/− animals exposed to the cell-impermeant carbonic anhydrase inhibitor benzolamide (BZA) or the cell permeant, broad spectrum inhibitor dorzolamide (DZA). (b) Quantitation of carbon dioxide responses in wild-type and Car4^−/− animals; mean ± sem (n=6). Green bar denotes wild type responses to 30% CO₂ in the presence of BZA. See Supplementary Fig S5 for responses to other taste stimuli.

Figure 4. Requirement of PKD2L1-sour cells for the taste of carbonation

Represeantative integrated chorda tympani responses to sour, sweet and CO₂ stimuli in wild type, or in animals expressing TeNT in PKD2L1 sour-sensing taste receptor cells. See Supplementary Fig. S5 for responses to additional tastants and quantitative analysis.