Are you sure about what you are tasting?

Day 21 in our ChristmasCalendar2019!
Food: anything with a taste

Christmas is (almost) here! In Sweden, Christmas is a time not only for love and fun, but also for the bliss of taste. A Swedish Christmas is never complete if you have not yet experienced the warmth of glögg, the delicacy of pepparkakor, the joy of julmust, and the cosiness of julbord.

All of these would be deeply missed if we lose the ability to taste. Thanks to the taste buds on our tongue and palate, which grant us the sense of sweet, sour, bitter, salty and umami. They surely guide our way through the julbord buffet!

What is taste?

Taste receptor cells (TRCs) are the basic taste detecting units. A taste bud contains around 50-100 TRCs, and there are hundreds to thousands of these buds scattered around in the mouth, each connected to a collection of neurons that eventually feed into the taste processing centre in the brain (gustatory cortex).

Substances that trigger taste perception, or tastants, are a broad spectrum of chemicals that have evolutionarily attracted our brain’s attention, usually because they just happen to be either useful or poisonous.

For example, salt is so important for our health that we evolved a specific sensor (an ion channel known as ENaC) solely for sodium ions on our tongue (1). It makes sure that we crave for it, but hopelessly regret if we swallow a spoonful of salt. The same goes with deadly poisons. Cycloheximide and cyanide, for instance, stop us from making our essential proteins and ATP. Therefore, the body decides to assign a bitter taste to them to save us from dying a horrible death.

Taste perception is hardwired

Unlike what is popularly believed, taste buds are not distributed according to the classic ‘tongue map’ where each region responds only to one specific taste. Instead, each taste bud contains TRCs for all the five tastes, although sensitivity to each taste modality does seem to depend on its location in the oral cavity.

But don’t taste buds get confused detecting different things at the same time? They are actually smarter than we think. Most TRCs on a taste bud is dedicated to perceiving one single taste modality. That means when a ‘sweet’ cell in the taste bud gets activated, we only sense sweetness. To make this work, the ‘sweet’ cells contain only receptors for sugars, and ‘bitter’ cells only have bitter receptors. 

Simplified model of a taste bud containing TRCs.
Fun fact: although the salty sensor protein is now known, a salty cell is yet to be identified.
Diagram by Kelvin Kwok

What happens if we substitute the sugar receptors in the sweet cells with bitter receptors? Apparently, we will fall in love with bitter food (hopefully not poisonous) and start munching on it like a sugar cube. And if we do the opposite, we will probably abandon all our beloved desserts. This concept has already been proven in mice and insects carrying similar mutations (2-4). In theory, if we substitute the sugar receptor with a blue light receptor in the ‘sweet’ cells, we would feel like swimming in the sea of sweetness when looking at blue light with our mouth wide open (5). This decoupling of tastant and taste perception is interesting yet strange. Who knows what your colleague is going through drinking that glass of fine wine you really enjoyed!

Tasting to survive

Taste, after all, is a perception. Something that our body pragmatically developed over millions of years, in order to guide our behaviour and achieve what is the most evolutionarily beneficial to us.

Insects would agree with that. Studies have shown that their taste perception is even more strongly coupled with behaviours. They also have more peculiar taste modalities which appear to be crucial for their unique mode of survival, including feeding and breeding.

Butterflies have sweet receptors on their legs, so that when they land on a flower, they will know immediately if any nectar is available and of what quality. To further facilitate their decision to feed on it, sensing sweetness on the leg causes them to reflexively extend their straw-like mouth into the nectar and start drinking, without giving too much thought (6). This feature is shared by the fruit fly. But more careful examination revealed an additional set of neurons connected to sweet receptors on their legs, which demands the fruit flies to stop moving once they land on something sweet (7). By that, the fruit fly’s taste system has developed a double safeguard to make sure the flies feed on as much sugar as possible.

The taste of water

Breeding is equally important. In order to lay eggs in the right place, a deadly mosquito species that transmits Zika and dengue fever may have acquired a peculiar sense of taste. These mosquitoes lay eggs only in freshwater, because their larvae cannot survive in salty seawater. We know these mosquitoes always dip their legs in the water before laying any eggs. But how do they distinguish between the different water sources?

Recently, scientists discovered sensory neurons on the tip of their legs (tarsus) that express a sensor protein called ppk301 on their surface (8). Without a functional ppk301, mosquitoes start laying eggs randomly in fresh or salty water. But, intriguingly, when scientists thought this receptor simply detects the saltiness in water just so the mosquito avoids it, they realised that ppk301 is activated even by deionised water, albeit at a different intensity. Hence, it seems that one receptor for water is capable of causing bimodal perceptions. Scientists postulate that a permission to lay eggs comes from the taste of water, which can be overridden by excessive salty taste in it.

Taste on another dimension

Although many mysteries regarding taste have been deciphered, there are many more questions. Like the case of water-tasting mosquitoes, taste systems are certainly much more complex than we thought. Indeed, the ‘labelled line’ concept depicting one-taste-cell-for-one-taste-modality is increasingly challenged by an alternative model, where taste perception is informed by the combinatorial activity of multiple taste cells that respond differentially to all ranges of tastes (9).

That might explain the much richer experience we enjoy when various tastes are skillfully partnered up in a luscious meal, rather than having discrete tastes trying to tango awkwardly in our brain.

Enjoy your tasty Christmas meals!

(1)  Nature 464, 297-301 (2010)
(2)  Cell 115, 255-266 (2003)
(3)  Nature 434, 225-229 (2005)
(4)  Neuron 49, 285-295 (2006)
(5)  Cell 139, 234-244 (2009)
(6)  Journal of Insect Physiology 54, 1363-1370 (2008)
(7)  Nature Communications 7, 10678 (2016)
(8)  eLife 8, e43963 (2019)
(9)  Nature Reviews Neuroscience 18, 485-497 (2017)

Feature photo from pixabay.com

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