What tastes good to you right now? A sweet carrot? A bitter beer? Or maybe some salty pretzels? Whatever it is, how it tastes to you is determined by your cells. So how do we taste?
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We separate tastes for humans (and probably most mammals) into five categories: bitter, sour, sweet, salty, and umami (the taste generated when you consume certain amino acids, such as MSG). Our taste preferences when we are born are genetically encoded. We prefer sweet tastes as babies, because “sweet” is associated with healthy foods that contain proteins and energy. We avoid bitter and sour things, because these are associated with toxins and acidic foods, like spoiled fruits or harmful plants. Humans (and mice) generally prefer salty foods when our bodies are low in sodium with our tastes fitting our physiological need. As we mature (or most of us anyway), we acquire tastes for more bitter and sour things, such as coffee and lemons, and may prefer salty and sweet foods even though we really don’t need to build up our salt or sugar supply.
From studies in mice, it is thought that the five tastes are determined by separate taste-receptor cells (TRCs) in the mouth each tuned to a specific taste. TRCs are organized into taste buds, composed of 50-100 cells, and these taste buds are housed within papillae (the bumps on your tongue). Taste buds from all regions of the mouth contain cells that respond to the five tastes and are connected to nerves that carry taste information to your brain stem and into the primary gustatory cortex (where your response to a food and your perception of flavor is dictated). Contrary to popular belief, there is no “taste map” on the tongue.
Recently published online in Nature, Jayaram Chandrashekar and colleagues (including scientists from the National Institute of Health in Bethesda) illustrate how salty taste occurs in sodium sensing in mice. Each TRC that determines a salty taste contains a sodium channel (or sodium transporter). This channel is present in many cells within your body, including your kidneys, lungs, and sweat glands, and is important in salt transport. Its role is so important that if this channel is made completely nonfunctional in mice, death occurs soon after birth. In order to study the importance of this channel in taste, scientists used a clever technique (attaching the channel mutation-causing agent to a known TRC-specific gene) to generate mice that were missing the sodium channel only in their taste buds. Unlike their ‘normal’ peers, these mice displayed a complete loss of salt attraction and sodium taste response while responding normally to the other four tastes. And most importantly, all of the TRC cells that carried the mutated salt channel did not carry any markers for the other four taste sensors, showing again that individual TRC cells are only able to determine one taste.
Whether or not this specific sodium channel is as important to salty tastes in humans as it is in mice is unknown. Due to our molecular similarities, the authors note that it is likely. But unlike mice, our innate responses to salty taste may be overridden by our high-salt diets.
Now, pass me those fries. All this talk of salt has made me hungry.
So the DNA in the taste buds of these mice is different than the DNA in the entire rest of their bodies?
First, a little background: DNA is composed of discrete sequences called genes. Genes can be expressed into proteins via RNA. Each gene has a region preceding it called a promoter. Different factors act on these promoters to either inhibit or elicit gene expression into protein (such as the salt channel). In the most simplistic sense, promoters control the expression of a gene into protein.
In these mice, the DNA at the very beginning of their embryonic development was all the same. Then, as the embryo grew, the cells separated into specific tissues, and this is when the DNA changed. In the Cre-Lox technique, a gene (called Cre) encoding a protein that can cause the mutation of one specific target sequence (such as the gene encoding the salt channel) is placed after a promoter that regulates tissue-specific gene expression. So in this case, Cre was placed under the control of a promoter that only turns on in TRC cells. When Cre was expressed into protein, it was only in these cells. Once it was expressed, it acted on the salt channel DNA specifically and caused mutation, so that the gene expressing the channel only changed in TRC cells.
So in the live mouse, the DNA sequence of the TRC cells differs from the DNA in all the other cells due to tissue-specific expression of the introduced Cre protein.
very tasty!
No taste map, eh? Good to know! I had thought the opposite was true.
In the words of Homer Simpson, “Mmmm, I taste good!”
So how does our tongue know how to taste, does the tongue pick up/measure certain aspects of something? e.g. if I have never tasted something before e.g. a leaf from the amazon, how does the tongue pick up the taste?
peter — The components of the things we eat interact with the taste receptors on our tongue. So if you eat an amazonian leaf, the compounds present on the leaf make contact with your taste receptors, and depending on what those components are, a signaling pathway can be activated. For example, if the leaf has some sodium chloride on it, your sodium channel taste receptor would bind to the sodium and transport it, resulting in a signal being sent to the brain. If there was a certain type of protein (like a toxin), that protein may bind to a different taste receptor and initiate a different kind of signal. It’s all about substances interacting in different ways with your taste receptors. It doesn’t matter if you’ve never tasted a leaf, the chemical components in that leaf are still detected by your tongue and may still bind to your specific taste receptors.
Thanks amanda, its an amazing part of the body the tongue.
So is it possible to taste nothing then, say we tasted something that did not contain anything that the receptors could pick up, would it taste nothing, it seems when I taste something that would not likely have something the receptors would pick up, I still taste something.
peter — I guess theoretically, yeah, if there was absolutely nothing in the substance that activated your taste buds, there would be nothing to taste. But there is always *something*, I would imagine. If certain taste receptors could respond to temperature or texture of a material, you may \taste\ those things as well. I don’t know enough about tongue receptors to know the full extent of everything we can taste, but I imagine everything we put in our mouths has some component our receptors can respond to. And I’m not sure about receptor sensitivity, either. Is a molecule of Carbon enough to activate a receptor? It is intriguing.
[On a side note, I've always wondered the same about smell. I know next to nothing about it, and I always wonder about tiny particles of perfumes or pheromones entering the nose and eliciting whatever response it is they elicit. Amazing, really. But that's for another post someday.]