tasting sour on your tongue and in your spine

Here's another intruiging science-of-food result, published in last week's Nature. (Link may require Nature subscription, sorry.) A team of scientists in San Diego wanted to figure out exactly how we taste sourness, such as the fantastic acidic taste of a white balsamic vinegar. People already knew how sweet, bitter, and umami tastes work, what chemical receptors are triggered when each of those flavors touch your tongue, but sour and salty receptors had yet to be identified. The scientists figured it out, and made a second discovery that it seems to me might suggest how the ability to taste sourness could have evolved. The same chemical receptor that detects acidity (sourness) on the tongue also detects acidity in the cerebrospinal fluid around the spine, and transmits that information to the brain.

The researchers proceeded by first using information about where different proteins are physically located in the body, then used genetically-modified mice to confirm their result. They initially found in a bioinformatics database about 30,000 proteins that are expressed on the outside of cells, and could thus interact with the chemical environment outside the cell.

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... 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).

That is, mice without this protein, the only protein that is expressed on the surface of a cell, that is present on the tongue, and that is not present in other taste buds, these mice acted exactly like you would expect from mice that could not taste sour things. They tasted everything else like normal mice, but without the cells with this one protein, they couldn't detect the acid levels in food.

They then wondered if that same protein was used anywhere else in the body. They used a staining technique to find out that the only other place where this acid-detecting protein was expressed was in neurons surrounding the spinal cord. This is interesting. It seems like that same protein may be used to detect deviations from the proper pH (acid/base levels) in the spinal fluid. Low-level parts of the brain stem control a large number of biological processes in the body, such as temperature regulation and the regulation of chemicals and hormones. If levels of chemicals go out of balance, you could get very sick or die, and in some cases imbalances are consciously perceived as feelings of malaise or internal pain. Some parts of the system that regulates pH in the blood and spinal fluid had previously been known, but not the precise receptor in the spinal column. Now, after this research on taste buds, it's known. (At least for mice.)

The researchers don't seem to explicitly mention it, but this result seems to hint at a mechanism for the evolution of the ability to taste sourness. The regulation of pH in the spinal column is doubtless a very old adaptation in vertabrates, and it would probably not be a surprise to find out that a very similar acid-detecting protein exists in a large number of animals with nervous systems. However, the taste of acidity might well be a much newer adaptation, perhaps due to some sort of poison or irritant in food. You can easily imagine a population of ancient animals that would suffer from eating acidic food, but have no ability to detect sourness. A mutation that caused the already-existing PKD2L1 protein to be expressed in some taste buds would cause an immediate selection advantage, and the mutation would quickly spread throughout the population, where it remains today, an exaptation that allows us to taste lemonade.