Liman Lab Reveals Mechanism of Sour Taste

Summary

The Liman Lab at USC has confirmed that OTOP1 is the critical sensor for sour taste perception in mice.

Abstract

The Liman Lab, after years of research, has identified OTOP1 as the protein responsible for detecting sour taste in mice. This discovery was facilitated by advances in gene expression analysis and genetic engineering. OTOP1 is an ion channel that specifically passes protons, aligning with the acidic nature of sour substances. Mice engineered without OTOP1 showed a significant reduction in nerve activity in response to sour stimuli, confirming its role as a sour taste sensor. This finding adds to the understanding of taste perception and the molecular basis of how animals and individuals experience sour flavors.

Opinions

The Liman Lab's research is pivotal in understanding sour taste perception at a molecular level.
The study emphasizes the importance of OTOP1 in sour taste detection, distinguishing it from other taste sensations.
The work suggests that individual variations in sour taste perception could be due to differences in OTOP1 function or expression.
Dr. Liman highlights the significance of the discovery, noting it as a step forward in comprehending sensory perception diversity among animals and humans.

Lab at USC definitively establishes the proton ion channel OTOP1 as a sour taste sensor.

The protein responsible for detecting sour in taste buds has been difficult to identify. This is partly because sour chemicals are detected both through the sense of taste (taste perception) and through the production of pain (somatosensory perception). Thus, simple taste preference tests with mice are inconclusive. Even without the taste pathway, mice still avoid sour-tasting liquids. Advances in the technologies that enable researchers to determine gene expression and perform genetic engineering mice have led to the discovery of the sour taste sensor.

The Liman lab has been studying sour taste for many years. In 2018, her lab identified a candidate sensor for sour tasting-chemicals, the protein otopetrin 1 (OTOP1). This year, her lab provided key evidence that OTOP1 is a critical sensor for the tongue to signal the brain about sour tastes.

Sour is one of 5 tastes: Sour, sweet, salty, umami, and bitter. The receptors that bind chemicals for the other five tastes are all known, but the receptor or sensor for sour-tasting chemicals has been elusive.

Taste occurs when chemicals wash across specialized cells in the mouth and on the tongue. Taste buds contain different taste receptor cells that respond to the different chemicals and release neurotransmitters that activate special nerves, called gustatory nerves, to tell the brain whether the chemical has a bitter, sweet, umami, sour, or salty taste, or is a mixture of these. The proteins that sense the different tasting molecules are functionally and structurally diverse.

OTOP1 is a protein that forms an ion channel at the surface of the sour taste receptor cell in a taste bud. Ion channels pass positively or negatively charged atoms or molecules. OTOP1 is a proton channel, meaning that it passes H+ ions into the cell. This makes perfect sense for a sour taste sensor, because sour chemicals, foods, and drinks (like lemons, lemonade, or vinegar) are acidic. Acids are low pH, which means that they have high concentrations of protons.

The Liman lab determined that the sour taste receptor cells from mice without OTOP1 did not transport protons when exposed to acidic solutions and did not become excited enough to release neurotransmitters in response to acid solutions. When they placed sour solutions into the mouths of the OTOP1-knockout mice, the activity of the gustatory nerves was severely reduced. Thus, her lab has now definitively established OTOP1 as a sour sensor.

Find out how the tongue responds to sour tastes in the Sour Taste Perception Video

With such detailed molecular knowledge, we can begin to understand why some people find sour so unpleasant and why others enjoy sour tastes. As Dr. Liman says

Finding the molecular basis for the sour-taste sensor takes us one step closer to understanding how different animals and individuals perceive the world.

Highlighted Research

Y. H. Tu, A. J. Cooper, B. Teng, R. B. Chang, D. J. Artiga, H. N. Turner, E. M. Mulhall, W. Ye, A. D. Smith, E. R. Liman, An Evolutionarily Conserved Gene Family Encodes Proton-Selective Ion Channels. Science 359, 1047–1050 (2018). DOI: 10.1126/science.aao3264