A taste for sour fruit: our tongues have specific receptors to detect acid.Getty

Researchers have worked out how a mammal's tongue detects sour tastes: it's all down to a single, specialized receptor, they say.

Taste in mammals is classified into sweet, salty, bitter, sour and umami (the taste of monosodium glutamate, commonly found in Chinese takeaways). Until now, only the sweet, bitter and umami taste receptors had been identified, and researchers were unsure whether the other two tastes had specialized receptors for them at all.

The three tastes with known receptors are triggered by large molecules, such as sucrose, that latch on to and are recognized by specialized cells on the tongue. But salty and sour are different in that they are the tastes of very simple ions: hydrogen ions (H⁺) for acidity and, mainly, sodium ions (Na⁺) for salt. Some researchers have speculated that many cells in the tongue might be able to pick up these signals, relaying the information in a complex pattern of nerve signals to the brain. "This kind of model is very messy," says Charles Zuker of the Howard Hughes Medical Institute in San Diego.

So Zuker's team — the same lab that pinned down the previous three taste receptors — set out to hunt for a sour taste receptor.

Angela Huang, a graduate student in Zuker's lab, first trawled through the mouse genome to pick out any proteins that exist in cell membranes: proteins that can pick up signals from the outside world and transmit them to nerves. That left about 10,000 candidates.

They screened these by assuming that a taste receptor would only be found in a small number of tissue types (specifically tongue taste cells). That whittled the list down to 900. They then looked for gene patterns known to exist in other taste receptors, leaving a single protein called PKD2L1 as a prime candidate.

To check on the action of PKD2L1, the team created genetically engineered mice that produced a toxin in cells expressing PKD2L1, killing these cells. Probes placed inside the mouse brains then showed that no neural activity was prompted by sour-tasting foods in these mice, they report in Nature¹. And their behaviour changed to match: they kept licking sour foods, whereas normal mice would run away from acidic snacks (only humans have a taste for sour foods; other animals avoid them).

Sweet success

Zuker's team also hit upon a surprising fact about the sour receptor: it seems to show up in neurons of the spinal cord. "This is the first time that a taste receptor has been shown to respond to stimuli in another part of the body," says Zuker. This 'taste' sensor might help the body to monitor acidity in the nervous system, he says.

Another group of scientists who were similarly on the trail of a sour-taste receptor also hit upon PKD2L1 as a candidate. Hiroaki Matsunami from Duke University Medical Center in Durham, North Carolina, and colleagues showed PKD2L1 and PKD1L3 being activated by acid in mouse cells in the lab dish². They also found that these proteins were well positioned on the tongue for contact with food, but were unable to confirm that there was just one dedicated receptor for sour taste. Zuker's work fills that gap.

The two studies together certainly seem to point the way to understanding sour taste, says Gary Beauchamp of the Monell Center in Philadelphia, Pennsylvania.

Taste test

But it seems that not everything is understood. Strangely, while taste is an instantaneous perception, Matsunami's work showed a delay between the introduction of acid and the cells firing off a 'sour' signal, says Zuker. This indicates that something else might be going on inside the mouth to help mammals identify the taste.

Beauchamp adds that it is also unclear how or why a sour receptor would come to be. There is a clear evolutionary motivation for the existence of some other taste receptors: bitterness detects poison and sweetness detects sugar, an essential source of energy. "It is still not entirely convincing why we need a sour taste receptor," says Beauchamp. "None of the suggestions for sour taste, such as being able to detect unripe fruits, are entirely compelling."

For Zucker's team, what comes next is a search for the salt receptor. "It's just a case of going through those proteins that are left behind when all the other taste receptors are gone," says Zuker.

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• References

  1. Huang A. L., et al. Nature, 442. 934 - 938 (2006). | Article |
  2. Ishimaru Y., et al. PNAS, 103. 12569 - 12574 (2006).