Sour Research, Sweet Results

In a paper published on Nov. 24 in the Proceedings of the National Academy of Sciences journal, USC College’s Emily Liman reveals the physiology behind all the puckering — how people taste sour.
ByPamela J. Johnson

This Thanksgiving, when you nibble the cranberry sauce and the tartness smacks your tongue as hard as that snide comment from your sister, consider the power of sour.

Of the five taste sensations — sweet, bitter, sour, salty and umami — sour is arguably the strongest yet the least understood. Sour is the sensation evoked by substances that are acidic, such as lemons and pickles, and the more acidic the substance the more sour the taste. But how acids, and the protons they release, activate the taste system has been beyond comprehension.

Emily Liman, associate professor of neurobiology in USC College, and her team have discovered one way that cells responsive to sour tastes detect protons.

They expected to find sour protons binding on the outside of the cell opening a pore in the membrane that allowed sodium to enter the cell, producing an electrical response. That electrical response would be transmitted to the brain.

Instead, they found that the protons released by sour substances were not binding to the cell’s exterior but were entering the cell. Their research revealed that it is the entry of the protons into the cell that causes the electrical change. Liman’s research was published and highlighted on Nov. 24 in the Proceedings of the National Academy of Sciences (PNAS) journal. The paper was co-written by neuroscience Ph.D. student Rui B. Chang and research specialist Hang Waters, the latter now at the National Institutes of Health.

“In order to understand how sour works, we need to understand how the sour-responsive cells detect the protons,” Liman said. “In the past, it’s been difficult to address this question because the taste buds on the tongue are heterogeneous. Among the 50 or so cells in each taste bud there are cells responding to each of the five tastes. But if we want to know how sour works, we need to measure activity specifically in the sour sensitive taste cells and determine what is special about them that allows them to respond to protons.”

Liman and her team created genetically modified mice and marked their sour cells with a yellow florescent protein. Then they recorded the electrical responses from just those cells to protons.

The ability to sense protons with a mechanism that does not rely on sodium entry has important implications for how different tastes interact, Liman speculates.

“This mechanism is very appropriate for the taste system because we can eat something that has a lot of protons and not much sodium or other ions, and the taste system will still be able to detect sour,” she said. “It makes sense that nature would have built a taste cell like this, so as not to confuse salty with sour.”

In the future, the research may have practical applications for cooks and the food industry.

“We’re at the early stages of identifying the molecules that contribute to sour taste,” Liman said. “Once we’ve understood the nature of the molecules that sense sour, we can start thinking about how they might be modified and how that might change the way things taste.  We may also find that the number or function of these molecules changes during the course of development or during aging.”

Read more articles from USC Dornsife Magazine’s Spring/Summer 2011 issue