When a flavor hits the tongue, it activates a taste cell — for sweet, salty, bitter, sour or umami (savory). That taste cell then passes the delicious message on to a brain cell so your brain knows whether the mouth is tasting cake or steak. When those taste cells die — and they do — new ones take their place. How do those new cells find the right brain connections without getting mixed up? They send a specific, irresistible chemical summons, a new study shows.
Your tongue is a constantly changing landscape. The surface is covered in taste buds, and each bud is filled with a mix of different types of taste-receptor cells. The base of each taste cell in the bud is linked to a long tail called an axon. That axon is part of a brain cell, or neuron, located in a bundle of cells just behind your ear. This cell bundle takes the information from each taste-receptor cell and passes it along. This allows our brains to taste the difference between apples and anchovies.
The cells in your taste buds make direct contact with everything that ends up in your mouth. Along the way, they may get burned or poked or bitten. So each cell only lives a short time. How short? Perhaps only about one to three weeks. As the cells die off, new ones emerge to take over for the old ones.
Those new cells have to hook up to the proper neurons. “You don’t want bitter-taste-receptor cells hooking up with a sweet neuron,” explains Hojoon Lee. He is a neuroscientist — someone who studies the brain — at Columbia University in New York City. New sweet-taste cells need to link to sweet neurons. Bitter ones must join up with bitter neurons. If, by accident, some bitter-taste-receptor cell did link up to a sweet neuron, Lee says, the animal or person might mistake bitter things — which often are poisonous — for a harmless sweet food. And that could be a potentially deadly mistake.
How do the new taste-receptor cells find the right neurons? “I thought there must be something those taste receptor cells had which ensured miswiring wouldn’t happen,” Lee says. To find out what it was, he and his colleagues removed taste-receptor cells for sweet and bitter from mice. Then they looked at what genes — or sets of instructions — those cells had been using to function.
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Hacking cellular communication
The scientists hoped to find instructions for signaling particular neurons. And soon they hit upon a group of molecules called semaphorins (Seh-mah-FOR-ins). These chemicals are named for semaphores. These are systems in which people communicate by moving brightly colored flags. Those flags, and their messages, can be seen from far away. Semaphorin molecules aren’t flags. But cells use these chemicals to communicate with neurons, attracting some and repelling others.
Bitter-taste cells made a lot of one type of semaphorin, known as 3A. Sweet-taste cells made a lot of type 7A. To find out if each type summoned different neurons to the taste cells, Lee and his colleagues created something called a knockout mouse. This mouse had a special gene that would make bitter-taste cells delete semaphorin 3A if the mouse received a certain drug. After this happened, the researchers watched as the new cells tried to hook up with the correct neurons.
Without semaphorin 3A, bitter neurons seemed to have lost their roadmap. Nearly half inappropriately hooked up with salty-, sweet- or umami-taste cells. Oops.
“We saw that bitter neurons were making more mistakes and going elsewhere,” Lee says. “We took this to mean that 3A is the guiding signal that brings the bitter neurons to the bitter-taste-receptor cells.”
But they didn’t know that for sure. Semaphorin 3A could have been attracting the bitter neurons. Or they might instead have pushed away sweet, salty, sour and umami. That would have left bitter as the only receptors left to match up with them. To find out whether the semaphoring actually called out to bitter neurons, the scientists performed another experiment. They made the sweet- and umami-taste cells in other mice inappropriately produce semaphorin 3A. Now the bitter neurons came racing to hook up with sweet and umami cells.
When Lee and his colleagues did the same studies with semaphorin 7A, they got similar results. Sweet neurons responded to sour-taste cells that were making semaphorin 7A.
The scientists shared their findings August 9 in the journal Nature.
“Among the five senses, taste is probably the least known or examined,” says Yuki Oka. He studies the brain at the California Institute of Technology in Pasadena. He was not involved in the Lee’s study but finds it “very elegant.”
While the studies were all done in mice, there’s no reason to think human taste cells would be too different, Oka says. “It’s the same system,” he explains. Any molecule involved might be a little different, “but it’s likely to be similar.”
Lee and his colleagues so far have found semaphorin molecules only for sweet and bitter. They are testing others for sour, salty and umami cells. But there’s probably far more than just one molecule controlling the final hookup, Lee points out. Why does he say that? Even when the bitter or sweet taste cells had no semaphorins, the neurons still found their way to the proper taste receptor about half the time. This means that other molecules probably help the neurons find their way. After all, when it comes to tasting the difference between poisons and pie, you can never have too much insurance.
activate (in biology) To turn on, as with a gene or chemical reaction.
axon The long, tail-like extension of a neuron that conducts electrical signals away from the cell.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
colleague Someone who works with another; a co-worker or team member.
flavor The particular taste associated with something that is eaten or drunk. This is based largely on how it is sensed by cells in the mouth. It can also be influenced, to some extent, by its smell.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
knock out (in genetics) To remove or disable a particular gene. The term gets its name because the function of this gene has been knocked out by the procedure. Scientists use this method to find out what the function of a gene might be — by finding out what an organism is like without it.
knockout (in genetics) The term for an organism that has been bred or engineered in such a way that one of its genes has been disabled, or turned off. The term gets its name from the fact that the function of this gene has been knocked out by the procedure. Scientists can now identify the function of the missing gene by seeing how a cell — or organism — differs when this gene no longer works.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
neuron An impulse-conducting cell. Such cells are found in the brain, spinal column and nervous system.
neuroscientist Someone who studies the structure or function of the brain and other parts of the nervous system.
receptor (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.
taste One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).
taste buds A collection of 50 to 100 or so taste receptors. They’re found on the tongues of land animals. When certain chemicals in food or other materials trigger a response in these receptors, the brain detects one or more flavors — sweet, sour, salty, bitter or umami.
umami One of the five major tastes (along with sweet, sour, salty and bitter). It has been described as savory but most people find the mild flavor hard to characterize. It is particularly prized as a flavor in Japanese cuisines.