Discovery of a New Taste Receptor

Post by Anastasia Sares

What's the science?

Our sense of taste allows us to gain useful information about food and what it offers us nutritionally. A sweet taste signals the presence of quick energy in the form of sugars, and umami indicates proteins, while bitter foods are potentially toxic. So far, scientists have found that there is a dedicated receptor cell for each kind of taste (with the exception of some cells that detect both sour and salty elements). However, recently in PLOS Genetics, Banik and colleagues identified a brand new type of taste receptor: a broadly responsive (BR) cell that can detect more than one type of taste compound, and sometimes many at once.

How did they do it?

The authors had observed in a previous study that BR cells seemed to respond to multiple tastes. Taste cells can be tested using calcium imaging, because the cell releases calcium internally as part of its signaling process. First, a taste cell is isolated and suspended in a fluid that keeps it alive, and a dye is injected that binds to calcium. Under a special microscope (a confocal microscope), it is exposed to a laser that excites the dye, causing it to give off light that the microscope can detect. Then, the cell is exposed to different taste compounds (sugar, acid, salt, etc). If it responds by releasing calcium, the image will get brighter. Once they confirmed that these kinds of cells exist, the authors also performed a number of tests to understand how they work. By using genetically engineered mice with different proteins knocked out (or removed from the animal’s genetic code), they could determine the inner workings of the BR cell.

What did they find?

Previously-identified cells come in two main varieties: type III cells, which detect salt and acidity, and type II cells, which each detect one of either bitter, sweet, or umami tastes. Because saltiness and acidity are both the result of ions in solution, type III cells have ion channels at their surfaces to detect these tastes. Type II cells are a bit more complicated, each one with a specialized protein on their surface to detect a single sweet, bitter, or umami compound.

So how could some BR cells detect bitter, umami, sweet, and sour all at once? The BR cells seem to be a subset of type III cells that have both ion channels and complex proteins at their surface. Their mechanism depends on a protein called phospholipase Cβ3. When this protein was removed from the genes of mice, they responded much less to bitter, sweet and umami tastes, even with type II cells left intact.

What's the impact?

This work highlights the fact that taste perception is an evolving field: we still haven’t discovered even the most basic cells that contribute to our sensory abilities. It also reinforces an important idea that has proven true for other senses like vision and audition: we use a combination of highly specialized receptors and other “generalist” receptors to help us perceive the world around us.

Banik et al. A subset of broadly responsive Type III taste cells contribute to the detection of bitter, sweet and umami stimuli. PLOS Genetics (2020). Access the original scientific publication here.