Wednesday, January 22, 2014

A Taste Of Things To Come

Biology concepts – gustation, taste papilla, evolution, defense

The miracle berry is the fruit of the Richadella dulcifica bush
from West Africa. The plant also goes by the name Synsepalum
dulcificum. Why it would have two scientific names, I have
no idea. It has been chewed before meals in Africa for
hundreds of years. Now we have flavor-tripping parties
in the US to have fun with its properties.
Miraculin, what a great name for a protein! Of course, with a name like that it better do something pretty special. Miraculin is the active molecule in the Miracle Fruit, the favorite classroom activity of middle school science and high school biology classes everywhere. The Miracle Berry is the common name for the fruit of the West African plant, Richadella dulcifica.

For those of you who haven’t done this in class, here’s what happens. You eat the berry, and then try a slice of lemon. It tastes sweet! But the berry didn’t taste sweet when you ate it. Try a sour patch kid candy – it tastes sweet too! The effect lasts about an hour and it feels weird; your brain expects one thing yet experiences another – it’s like an optical illusion for your mouth. Biologically, this is a lot of chemistry just for taste. You get the sugar, protein, fat, or salt from what you eat whether you taste them or not, so is it important to taste things?

It must be important to taste things, or else we wouldn’t do it. Gustatory sensation is more than just a little complicated at the cellular and molecular levels, so it must play an important role in the survival and evolution of many species, otherwise it wouldn't be worth the costs.

We see things to find food, avoid predators, or find mates. We hear things to localize predators/prey or to find our kin when we can’t see them.  Smell is central for communication amongst species (pheromones) and for sensing danger (like smoke). But these examples describe gaining information at a distance, and are important for communication and safety. Does gustation fit into any of these categories?

Tasting something can’t be done from a distance – humans have to stick the target in their mouths – so what’s the big deal? It’s important because your brain is asking the question, “Should I swallow what’s in my mouth?" "Is this O.K., or is it going to kill me?”

Evolution has honed our brains to crave those things we need and spit out those that will do us harm. Sweet foods translate as energy – your brain says, “Eat this, it has carbohydrates – you need those.” What would be the best way for your brain to convince you to eat what is good for you? It bribes you with a pleasant payoff; we perceive it as tasting good, and we want more.

The olfactory neurons stick through the cribiform plate
in the nasal cavity. These neurons are raw nerve endings,
with receptors for different molecules. As such, they are
the only place where your central nervous system comes
into direct contact with the outside world, and they have
more receptor diversity than any other part of the body.
Just think of how many different smells you can recognize.
There aren’t that many receptors; different combinations
give you different odors.
On the other hand, babies don’t like sour or bitter. In terms of evolution and survival, nature is telling us to stay away from these tastes. Plants that make toxic chemicals are often bitter, so our primitive brain tells us that bitter = poison.

Rotting foods are acidic; acids are often the by-products of contaminating bacteria and fungi. Therefore, our old brain tells us to stay away from sour (acidic) foods. The gustatory sense is definitely protective. As humans, we can use our large brains to evaluate other cues as to food safety, so we can learn to like bitter and sour tastes; most animals just go with what Mother Nature tells them.

Gustation is a direct chemosensory process. Molecules to be tasted must come into direct contact with the sensors (receptors) in the mouth. This is similar to your sense of smell, but with one distinction; the chemicals you smell are volatilized in the air. For example, you don’t smell a rose by sticking it up your nose, the rose scent molecules traveling in the air from the flower to your olfactory receptors high in your nasal cavity. Smell is distance chemo-sensing.

For taste, the target molecules to be sensed are carried in liquid, not air. You take a bite of something, chew it up to release some of the molecules, and they mix with your saliva. Saliva is more than 99% water, and it is the water they delivers the dissolved (water-soluble) molecules to your taste receptors. Our taste receptors respond to things that dissolve in water or fat. Things like vanilla, cinnamon and spices are not soluble in water, but they are in fat. Hurrah for fat!

On the left side are the different kinds of papillae on human tongues.
Foliate papillae are found only around the outside edge of the vallate
papillae. The filiform papillae do not have taste buds associated with
them. Note that how all the taste buds are located on the sides of the
papillae. This is where saliva will pool and be in place to activate the
receptors. On the right is a cat’s tongue. The filiform papillae are
longer and look like a comb, which is how they are used.
Notice that it isn’t our taste buds that sense the molecules. Taste buds don’t sense anything, they are just houses for our taste receptor cells. And the houses are located in neighborhoods called papillae. There are four types of papillae in the mouth; fungiform, foliate, circumvallate, and filiform. However, only the first three have taste buds associated with them.

Papillae are basically mounds of epithelial tissue that stick up from the surface of the tongue (see picture to left). Those with taste buds tend to be round or mushroom shaped, while filiform papillae are cone shaped and tend to point toward the back of the mouth. 

Filiform papillae are the most numerous, but are not directly involved in taste; they increase the tongue’s friction to help move foods toward the throat and to help break up food to release the taste molecules. In different animals they can have different shapes; in cats they are long and spindly, and though they feel like sandpaper, they are useful for grooming.

Taste buds are found only on the sides of the papillae; the food molecules must dissolve and move into the crevices between papillae. The taste buds themselves are small packages of gustatory receptor and supporting cells.

The cartoon shows how taste buds are constructed. Only the microvilli
are in the pore, so they “taste” the chemicals that reach the pore. At the
base of the receptor cells are attached to neurons. When a receptor cell
is depolarized along its membrane, it transfers that depolarization to
the neuron and an action potential is moved to the brain. The right is a
micrograph of actual taste buds. I thought it would be good to see the
real thing and compare it to the cartoon.
The receptor cells are housed below the surface, but have microscopic cytoplasmic projections (microvilli) that stick out of the gustatory pore and sample the chemicals that are washed over them (see picture). The taste receptor molecules are located all over the microvilli.

Each receptor corresponds to one taste sensation, so each receptor cell responds to just a single taste. It is the combination of all the receptors cells activated and the intensity of their activation that leads to complex tastes. We use to think that specific taste receptors were limited to certain areas on the tongue. But now we know that specific taste receptors are more concentrated in certain areas, but are present everywhere. For instance, you can sense sweet everywhere on the tongue, but sweet receptors are most concentrated at the tip.

When the particular tastant (the molecule that activates the receptor cell), fits into the taste receptor on the microvilli, it sends a single to a nerve which is embedded at the base of the cell. The more receptors that are engaged by tastant, the bigger the signal. This signal leads to a neural action potential that travels along the taste neurons to the brain, where they are converted to our sense of taste.

The receptors fit with their ligands (the tastant molecule) in a lock and key arrangement, although often more than one ligand will fit into a receptor. For example, the sweet receptor is a heterodimer (made from two different parts, called T1R2 and T1R3), and sucrose fits well into the receptors and is sensed as sweet. However, lactose (the sugar in milk) doesn’t fit as well, so it is sensed as less sweet.

Fructose is a great fit, so it is sensed as more sweet than sucrose. This is probably why high fructose corn syrup is added to everything today, it satisfies our craving for sweet better than regular sugar does. Artificial sweeteners are thousands of times sweeter than sucrose because they bind to the sweet receptor more tightly. Therefore, you can use a lot less of the sweetener than you would use of the sugar – and no (or few) calories.

This image is from the 2011 PNAS paper on miraculin
function. The two colors of the MCL protein in the top
cartoon represent the two shapes of MCL, one at neutral
pH and one at acidic pH. In the neutral pH conformation
(shape) the receptor is not activated, but it is occupied.
This is why you see in the bottom picture you see that
artificial sweeteners cannot activate the receptor after
the berry is eaten.
So we have two parameters that regulate our sense of taste; 1) how many receptors are activated at one time, and 2) how good a fit the molecule makes with the receptor.  So now that we know about taste receptors and action potentials, how might miraculin make sour things taste sweet?

A 2011 paper from the University of Tokyo has started to let us in on the secret. Remember that miraculin doesn’t make bitter things taste sweet, or salty things taste sweet, only sour – and sour things are acidic. It seems that it’s the acid that makes the difference. By manipulating the pH of the mouth, the researchers showed that miraculin has no flavor at neutral pH, but as the pH of the mouth decreases, the sweet taste increases.

So you eat something sour (acidic) after the miracle berry, and the pH of your mouth drops. Now the acid tastes sweet. The hypothesis is that miraculin binds to the sweet receptor in the lock/key fashion, but the shape of the protein doesn’t activate the receptor. But when the pH drops, the shape of the miraculin protein changes (protein folding is very much affected pH), and it activates the sweet receptor. This sends an action potential to the brain and we perceive sweetness. Anything that lowers the pH of the mouth is perceived as sweet. Sweet!

Gymnemic acid is isolated from another plant, but it suppresses the
sweet receptor action. The powder on the right is the isolated form, from
the plant, although it is not pure gymnemic acid. There are more
active compounds in the plant, but gymnemic acid is the most
potent one. It has been sold as a diet aid and now there is some
evidence that it may be active against diabetes.
There are other molecules that have the opposite effect. Eat some gymnemic acid for instance and then try a piece of chocolate. It won’t taste sweet at all - not a big money maker for the food industry. But there may be a way to use gymnemic acid. A study in India from 2012 says that treating early diabetic rats with gymnemic acid will even out swings in their blood sugar and will prevent early kidney damage. However, this effect may be due to the fact that gymnemic acid is an antioxidant, not to its ability to antagonize sugar receptors. Keep this in mind – we will come back to it in a couple of weeks.

We have more to say about our sense of taste, like what a supertaster is, and how we keep adding new tastes – no longer are we confined to just sweet, salt, sour, and bitter!

Koizumi, A., Tsuchiya, A., Nakajima, K., Ito, K., Terada, T., Shimizu-Ibuka, A., Briand, L., Asakura, T., Misaka, T., & Abe, K. (2011). From the Cover: Human sweet taste receptor mediates acid-induced sweetness of miraculin Proceedings of the National Academy of Sciences, 108 (40), 16819-16824 DOI: 10.1073/pnas.1016644108

Baig, M., Gawali, V., Patil, R., & Naik, S. (2011). Protective effect of herbomineral formulation (Dolabi) on early diabetic nephropathy in streptozotocin-induced diabetic rats Journal of Natural Medicines, 66 (3), 500-509 DOI: 10.1007/s11418-011-0614-y

For more information and classroom activities, see:

Taste sensation –

Miraculin –