Sour Sensing from the Tongue to the Brain

The ability to sense sour provides an important sensory signal to prevent the ingestion of unripe, spoiled or fermented foods. Taste and somatosensory receptors in the oral cavity trigger aversive behaviors in response to acid stimuli. Here we show that the ion channel Otopetrin-1, a proton-selective channel normally involved in the sensation of gravity in the vestibular system, is essential for sour-sensing in the taste system. We demonstrate that a knockout of Otop1 eliminates acid responses from sour-sensing taste-receptor-cells (TRCs). In addition, we show that mice engineered to express otopetrin-1 in sweet TRCs now have sweet cells that also respond to sour stimuli. Next, we genetically identified the taste ganglion neurons mediating each of the five basic taste qualities, and demonstrate that sour taste uses its own dedicated labeled line from TRCs in the tongue to finely tuned taste neurons in the brain to trigger aversive behaviors. Overall design: (1) Geniculate ganglion single cell sequencing. Previously our lab showed that the different taste qualities are mediated by dedicated TRCs tuned to individual tastes and innervated by dedicated ganglion neurons tuned to matching taste qualities (Barretto et al., 2015; Lee et al., 2017). We performed single cell RNA-seq on 800 neurons from the geniculate ganglion of P2rx3-tdTomato animals. We classified 454 neurons as expressing Phox2b, a marker for sensory neurons (D'Autreaux et al., 2011). From this dataset, we performed t-distributed stochastic neighboring embedding analysis (tSNE) to identify and visualize their distribution into different neuronal groups. Our results revealed 5 distinct clusters each potentially defining one of the five basic taste qualities, and 2 putative somatosensory clusters characterized by high levels of expression of the mechanosensory channel Piezo2. (2) Sour taste receptor cell bulk and single cell sequencing. To identify candidate sour taste receptors, we molecularly characterized the repertoire of genes expressed in sour (Pkd2l1-expressing) taste cells using a combination of bulk and single-cell RNA sequencing. We performed these sequencing experiments from animals expressing the tdTomato reporter in Pkd2l1-expressing taste cells and also carried out bulk RNA sequencing from “non-sour” TRCs and surrounding tongue epithelia to search for genes preferentially expressed in sour versus other classes of cells in the tongue.