Good Taste Is No Accident

USC College neuroscientist Emily Liman reveals a pathway from the tongue to the brain.
By Carl Marziali
Test tube studies have yielded contradictory results in the past, Liman said.

Want to take away that bitter taste in your mouth? Wish your children could stomach cod liver oil? New USC research unveils the workings of taste cells and suggests a way to make them work for you.

A group led by USC College neuroscientist Emily Liman has revealed the steps by which a taste cell goes from the physical capture of a “tastant” the crucial molecule of taste, such as sugar in cake to an electrical signal that tells the brain a treat has arrived.

The finding could influence efforts to improve the bitter taste of some drugs or to reduce the use of sugar by amplifying its effects on the brain.

Biotechnology companies are interested in changing the perception of taste, Liman said. And, she added, “how we are able to sense the environment is of fundamental interest.”

The group’s report appears May 23 in The Journal of Neuroscience.

Liman’s research centered on a gate in the taste cell, known as TRPM5 (pronounced “trip-em-five”) that allows electricity to pass between the exterior and interior of the cell and triggers an electrical impulse in the nervous system.

When a tastant binds to the taste cell, the gate opens and an electrical signal flows to the brain.

How exactly that happens has been a matter of debate. Test tube studies have yielded contradictory results, Liman said.

By comparing normal mice with ones lacking the TRPM5 gene, Liman’s group showed that the capture of a tastant starts a reaction that boosts the concentration of calcium within the taste cell. It is this increase in calcium that tells the gate to open, enabling the brain to perceive taste.

While this mechanism had been suggested previously, Liman’s study offers the first confirmation in taste cells.

“This signaling pathway is the critical pathway for these taste sensations,” Liman said.

The finding applies to taste cells for bitter, sweet and umami flavors (umami is the flavor of MSG). Still unclear is the pathway for cells that detect salty and sour flavors.

Curiously, the cells involved in vision and smell use different pathways. The closest relatives to taste cells are photoreceptors in flies, Liman said. Why this is has not been explained.

“My guess is that evolution is a sort of random walk that leads to unpredictable solutions,” Liman said.

The other authors of the report are USC graduate students Zheng Zhang and Zhen Zhao, and Robert Margolskee, professor of neuroscience at the Mount Sinai School of Medicine in New York.

Funding for the group’s research came from the National Institute on Deafness and Other Communication Disorders.