A new study suggests that individual cells in the tongues are exclusively dedicated to detecting sour tastes, a function that may have evolved to help mammals detect spoiled or unripe foods.
The finding contrasts with the notion that individual tongue cells detect more than one type of taste and send a complicated pattern of signals to the brain to determine which of five basic taste categories it fits in: sour, sweet, salty, bitter, or umami (the taste of monosodium glutamate, MSG).
Instead, researcher Charles Zuker of the Howard Hughes Medical Institute in La Jolla, Calif., says the sour receptors are found in a subgroup of taste receptor cells that do not detect sweet, bitter, or umami tastes.
The finding is "very interesting," says Zuker, in a news release, "because it seals the case that we had built before with sweet, bitter, and umami, showing that each taste is mediated by fully dedicated sensors."
Sensing Sour on the Tongue
In the study, published in Nature, researchers used DNA sequencing to isolate proteins involved in detecting sour tastes. Through a series of experiments they narrowed 30,000 candidates down to 900 possible proteins.
They were able to identify a molecule called PKD2L1 that was not found in cells that detected sweet, bitter, or umami. They tested it in an experiment using mice genetically engineered to lack this sour-sensing protein. Researchers recorded nerve signals and tongue function of the mice when they were exposed to a variety of tastes.
The results showed that no matter what sour tastes, like citric acid, they fed the mice, there were no nerve messages sent to the brain from the taste cells. But these "sourless" mice had no problems detecting sweet, bitter, salty, and umami tastes.
"Killing these cells and showing that the mice now are totally unable to detect sour proved that these cells are the sensors for sour taste, and that indeed no other taste cells detect sour," says Zuker.
Researchers say the sour-sensing PKD2L1 protein is also found in cells within the spinal cord and may help explain how the body monitors the quality of critical body fluids. For example, the body controls respiration in part by monitoring the acidity of the blood since an increase in carbon dioxide increases the acidity of the blood.
Defects in these sensing systems may underlie a wide range of disorders, says Zuker.
SOURCES: Huang, A. Nature, Aug. 24, 2006; vol 442: pp 934-938. News release, Howard Hughes Medical Institute.
By Jennifer Warner
Reviewed by Michael Smith