Two proteins – not one – could play a central role in the taste system, according to the findings of a University of Buffalo study.
The prevailing understanding of the taste system suggests the transduction of bitter, sweet, and umami stimuli in taste cells depends on G protein-coupled receptor signalling with transient receptor potential melastatin 5 (TRPM5) acting as a common downstream component.
In other words, the TRPM5 protein could be viewed as a gatekeeper for taste sensations. Without it, we would no longer be able to identify sweet, bitter or umami flavours
However, the Buffalo study, published in Proceedings of the National Academy of Sciences, finds that a second protein – TRPM4 – performs a similar role in the taste system.
In experiments on mice who had TRPM4, the rodents responded to taste stimuli, consuming sweet or savoury flavours “enthusiastically” while rejecting quinine, a bitter compound. Mice who were missing TRPM4 on their taste cells had more difficulty detecting sweet, bitter or savoury flavours.
“Our research shows that there is redundancy in the taste system,” said lead researcher Dr. Kathryn Medler, associate professor of biological sciences in the University at Buffalo College of Arts and Sciences. “This is important because taste is actually central to our survival. If you can’t taste something bitter, you might gobble up something that’s poisonous without ever knowing that it could be harmful."
TRPM5 doesn’t act alone
“Our study changes a central dogma in the field — that detecting bitter, sweet and umami stimuli is dependent on the presence of TRPM5 alone,” added Debarghya Dutta Banik, the study’s first author and a PhD candidate in biological sciences at the University at Buffalo.
Banik and Medler’s research concluded that mice were most sensitive to sweet, bitter and umami stimuli when the animals had both TRPM5 and TRPM4 on their taste cells. Removing either protein caused a decrease in sensitivity and removing both left the mice unable to detect flavour.
Like TRPM5, TRPM4 is a kind of protein called an ion channel. Found on taste cells, TRPM5 and TRPM4 channels open when sweet, bitter or savoury foods land on the tongue. This sets off a chain reaction of electrical signals carrying messages to the brain.
Relevance for human health
Though the study was done on mice, the research is likely relevant to humans, Medler said. TRPM5 and TRPM4 are both present in human taste cells, and TRPM5 is known to play a role in how people taste.
This is significant, the researchers suggested, because of the role taste has in determining diet and health. Perceptions of taste help to regulate appetite and may contribute to problems such as overeating and undereating, the researchers argued.
For instance, in a 2013 study, Medler found that the taste cells of overweight mice were less sensitive to sweets than the cells of slimmer counterparts. This indicates trouble detecting sweetness may lead obese mice to eat, she suggested.
Meanwhile, in humans, a loss of appetite is a common problem among older adults. Taste cells become less sensitive as people age, which can lead to a lack of interest in food, resulting in malnutrition and poor overall health.
“It’s important for us to understand how the taste system works,” Medler stressed. “The more we know, the easier it will be to find solutions to problems when the system isn’t working correctly.”
Source: Proceedings of the National Academy of Sciences
Published online ahead of print: doi: 10.1073/pnas.1718802115
"TRPM4 and TRPM5 are both required for normal signaling in taste receptor cells”
Authors: Debarghya Dutta Banik, Laura E. Martin, Marc Freichel, Ann-Marie Torregrossa and Kathryn F. Medler