Function, Innervation, and Neurotransmitter Signaling in Mice Lacking Type-II Taste Cells

The Skn-1a transcription factor (Pou2f3) is required for Type II taste cell differentiation in taste buds. Taste buds in Skn-1a(-/-) mice lack Type II taste cells but have a concomitant expansion of Type III cells, providing an ideal model to determine the relative role of taste cell types in response specificity. We confirmed that chorda tympani responses to sweet, bitter, and umami stimuli were greatly reduced in the knock-outs (KOs) compared with wild-type (WT) littermates. Skn-1a(-/-) mice also had reductions to NaCl that were partially amiloride-insensitive, suggesting that both Type II and Type III cells contribute to amiloride-insensitive salt detection in anterior tongue. We also confirmed that responses to sour stimuli are equivalent in the KOs, despite the large increase in the number of Type III taste cells. To examine their innervation, we crossed the Htr3a-GFP (5-HT(3A)-GFP) reporter mouse with the Skn-1a(-/-) mice and examined geniculate ganglion neurons for GFP expression and responses to 5-HT. We found no change in the number of 5-HT(3A)-expressing neurons with KO of Skn-1a. Calcium imaging showed that only 5-HT(3A)-expressing neurons respond to exogenous 5-HT, while most neurons respond to ATP, similar to WT mice. Interestingly, despite loss of all Type II cells, the P2X3 antagonist AF353 blocked all chorda tympani responses. These data collectively raise questions pertaining the source of ATP signaling in the absence of Type II taste cells and whether the additional Type III cells are innervated by fibers that would have normally innervated Type II cells.