In January 2018, a study published by the American Association for the Advancement of Science presented a surprising link between the sense of taste and the sense of balance. While trying to determine which genes are responsible for certain taste buds to ascertain sourness, scientists found the same gene at work in the inner ear.
When you think “sour,” you might think about puckering at the juice of a slice of lemon, but scientists think about pH levels. Sourness is actually a measure of acidity, due to the fact that a substance is acidic if it contains lots of H+ ions (hydrogen atoms with a positive electrical charge), which is also a mark of low pH. There are different kinds of taste buds: some recognize sweetness, some recognize saltiness, etc. The taste buds that recognize the sour *tang* of Sour Patch Kids contain ion channels that allow H+ ions to flow into the taste bud cell and send a signal to the brain that says, “Wow! This is sour!”
To ascertain which gene or genes are responsible for expressing the proteins necessary for building the H+ ion channels in sour taste bud cells, researchers at the University of Southern California used a mouse model. They compared the transcriptome of mice with sour taste buds with the transcriptome of mice without them. The transcriptome is a collection of all the RNA in a particular cell, and is an indicator of proteins that are being generated and built by a cell. When 41 potential proteins were identified in the sour taste bud cells, but not found in the other taste bud cells, the scientists knew one of them must play a role in the mechanism for detecting sour tastes.
The researchers implanted the potential genes into human embryonic kidney cells (HEK-293) or the female egg cells of a frog model (Xenopus oocytes). Then, the kidney cells and egg cells were flooded with an acidic solution and observed for H+ ion currents. The researchers noticed that the gene Otopetrin1, abbreviated as Otop1, was the only gene to produce an ion channel that permitted H+ ions to pass through.
The gene Otop1 is part of the otopetrin gene family, which happens to be known for the development and function of the vestibular system. The connection is clear when mice with Otop1 mutations exhibited issues with spatial orientation and balance. They could not properly right themselves or swim. Furthermore, the mice with Otop1 mutations had weaker currents of H+ ions in the taste bud cells, which suggests that the mice were not able to fully taste sourness. The scientists at USC hypothesize that Otop1 regulates an optimal pH level in the inner ear during development.
“We never in a million years expected that the molecule that we were looking for in taste cells would also be found in the vestibular system,” senior researcher Emily Liman said. “This highlights the power of basic or fundamental research.”
The Otop1 gene also produces H+ ion channels in the heart, uterus, adrenal gland, mammary gland, and in fat tissue, although the role of H+ ion channels in these regions is not understood. Further research may uncover more intriguing and unanticipated connections within our genetic makeup.
Tu, Y.H., Cooper, A.J.,Teng, B., Chang, B.R., Artiga, D.J., Turner, H.N., Mulhall, E.M., Ye, W., Smith, A.D., & Liman, E.R. 2018. An evolutionarily conserved gene family encodes proton-selective ion channels. Science [published online] DOI: 10.1126/science.aao3264.
Gersema, E. 2018. Surprising discovery links sour taste to the inner ear’s ability to sense balance. USC Press Room. Retrieved Feb. 18 from http://pressroom.usc.edu/surprising-discovery-links-sour-taste-to-the-inner-ears-ability-to-sense-balance/.