A Novel Family of Mammalian Taste Receptors

.	This site serves not only as a review of a journal article pertaining to the study of new information on taste receptors, but also as a collection of some basic information on taste perception and science. Without this integral information, the more advanced biological and genetic aspects of taste perception cannot be thoroughly understood. The paper "A Novel Family of Mammalian Taste Receptors" reports on the discovery of a new G protein-coupled receptors that the investigators refer to as T2R's. These receptors are highly involved in the perception of bitter taste. However, the T2R genes are only expressed when the their associated taste receptor contains the G protein alpha subunit gustducin. This makes sense as gustducin is a protein widely involved in certain taste perceptions. (Note: The name gustducin is derived from gustation being the sense of taste.) The investigators also looked at the genetic sequences of T2R's finding a common motif, as well as the possibility of that the receptor can recognize multiple tastants.	Figure 1. Mouth. Human mouth and tongue.	.
	Figure 2. The four basic tastes. The four basic tastes of sweet, sour, salty, and bitter have been given a general location on the tongue. Although these location are not regarded as concrete they serves as a simple model to understanding differently locations of tats perception. The fifth taste, umami, the flavor of MSG, is not represented in this figure.	Food is said to have five basic tastes: sweet, sour, salty, biter (see Figure 2), and umami (MSG). These tastes are sensed by the taste buds covering our tongue. Despite popular belief, the little bumps covering a tongue are not the taste buds, but rather the papillae. Fungiform and filliform papillae are on the front halves of the tongue, and foliate and vallate papillae are on the back. Filliform papillae have no taste buds. The taste buds cluster is packs of 2 to 250 within the papillae. On an even smaller scale, the actual buds contain about 50-150 receptor or basal cells. The average adult human has about 10,000 taste buds. There are also taste buds on the back of the mouth, epiglottis, and hard plate within the mouth.
	Taste sensation is realized is the mouth when the chemicals in food are dissolved by saliva. The free floating molecules are then able to enter a taste bud through the pore at its center (see Figure 5). The taste bud is activated by different mechanisms depending on the type of taste, which will be mentioned later, which then activates the appropriate cranial nerves (see Figure 3). The cranial nerves then send the signal to the solitary nucleus in the medulla (see Figure 4). The signal then travels upward via the medial lemniscus to the thalamus. The thalamus is a major relay station of the central nervous system. From the thalamus the signal of taste is sent to the hypothalamus, and more importantly to the somatosensory cortex where the taste is perceived.	Figure 3. The Cranial nerves involved in taste perception. There are three main cranial nerves involved in taste perception. The facial nerve innervates the anterior two-thirds of the tongue, the glossopharyngeal innervates the posterior third of the tongue, and the vagus nerve carries information for the back part of the mouth.
	Figure 4. Nerve path of taste sensation. Gustatory (taste) fibers synapse centrally in the medulla in the solitary nucleus. From there information is relayed to the somatosensory cortex and the hypothalamus via the thalamus.	It was mentioned before that the taste bud responds differently to different types of taste stimuli. Sweet and bitter tastes activate the chemical messenger gustducin. Gustducin begins an electrochemical cascade among receptor cells which ends in the basal cells at the bottom of the taste bud (see Figure 5), which activates the system described above. Sour and salty tastes are activated much more quickly. They rely on ion currents to conduct the signal of taste. It is for this reason that sour and salty taste buds are believed to be different than bitter and sweet taste buds. Sour and salty tastes are due to ionic gradients, whereas bitter and sweet tastes come from organic molecules. However, both systems end the release of neurotransmitter (see Figure 6).
	Research before the publication of "A Novel Family of Mammalian Taste Receptors" had identified just a few G protein-coupled molecules associated with taste transduction. Therefore, researchers had been looking at the possibility of the coupling of T1R molecules. However, even though then molecules were found in 30% of taste bud cells, they were not co-expressed with gustducin, a G protein alpha subunit. Therefore, Adler et al. decided to search for G protein coupled receptors in genomic sequences related to bitter taste perception. The importance of the biter taste is due to the idea that gustducin mediates bitter responses.	Figure 5. Taste bud. Diagram showing onion-like structure and components of a taste bud.
	Figure 6 (click to view larger image). Taste receptor cell. Diagram showing the components of a taste receptor with an emphasis on the ions and messengers involved. <credit: Taste & Smell>	The data provided in Figure 1 of the paper is the product of the rationale behind the role of gustducin in bitter taste perception. They reasoned that the difference in sensitivity to bitter substances reflects functional differences in the bitter taste receptor. Therefore, a DNA sequence database was searched for genes encoding transmembrane proteins at a location associated with bitter taste perception. The database search resulted in 23 amino acid sequences in three organisms: human, mouse, and rat. These novel, or new, amino acid sequences were identified as a class of receptors called T2R's. To see an example of a T2R sequence found in mice (T2R5), click HERE.
	Figure 1 of the paper takes the 23 amino acid sequences and aligns them for sequence similarity. The aligned sequences show 7 potential transmembrane segments, possibly involved in bitter taste perception. The unaligned regions may be regions needed to identify other ligands. Another fact to note is that T2R's do not contain introns. Figure 2 of the paper creates a phylogenetic tree/cladogram of the full-length human, mouse, and rat T2R's with opsin, and V1R vomeronasal receptors. Opsin and V1R are distantly related to G protein coupled receptors. The purpose of Figure 2 is to show that sequence similarity is high variable. Sequences range from highly related to highly diverted. However, what is important is that most T2R's share conserved sequence motifs in the first and last transmembrane segments. The variability is also not of a concern because that type of variable organization is also seen in olfactory genes. A cause of the variability may be due to the current quality of available database information not being perfect, or due to the presence of the pseudogenes discussed in figure 3. Figure 3 of the paper maps the specific loci on T2R genes that influence bitter taste. Chromosomes marked 5, 12, and 7 are human, and those marked 15 and 6 are mouse. The black lines with arrowheads are gene clusters found on the chromosomes. The arrowheads indicate the direction of transcription. There are two clusters of particular interest. The first is the bitter cluster found on the distal end of mouse chromosome 6. These four loci have been shown to influence response to bitter tastants. The four genes include Soa, Rua, Cyx, and Qui. What is of particular importance is that this bitter cluster corresponds to the 9 gene cluster on the distal end of human chromosome 12. The similarity between the two is particularly in the link to salivary proline rich protein (prp). Therefore, it can be concluded that T2R contains loci implicated in bitter perception. The researchers conducted further study on prp, but did not provide any data to analyze. Figure 4 of the paper serves to orient the reader to the location of the different types of papillae. The experimenters will be using an antisense RNA probe which has been shown to bind to circumvallate papillae in the posterior portion of the tongue. Remember that the posterior portion of the tongue is also associated with bitter taste perception.	Figure 7. Human mouth and tongue. Drawing of human mouth and tongue with photographs of fungiform papillae.
	Figure 8. Taste bud. Simple drawing of taste bud.	Figure 5 in the paper shows the expression of T2R's in various locations on the tongue using an antisense RNA probe. Frames a-e show that every circumvallate taste bud is expressing T2R in about 15% of cells. Frames f-h also show 15% expression, but in the foliate, geschmackstreifen, and the epiglottis. The contrast in this figure is frame i. This frame only shows a 10% expression, and is located in the fungiform. This demonstrates T2R content in this area.
	The purpose of the paper's figure 6 is to demonstrate each cells ability to express multiple T2R's. Frames a-c were probed with 2-10 different T2R probes. The result was not only an expression of T2R, but a slightly higher expression at 20%. However, there was not a higher incidence of expression in the fungiform cells. This indicates that there is little discrimination for the expression of T2R in the appropriate cells. And an example of this is that fungiform are not able to express T2R, therefore, we would not expect fungiform to play a role in bitter taste perception. But the other cells types expressing T2R most likely do play a role in this taste perception. Figure 7 of the paper is the summary of linking T2R gustducin. The fluorescent in situ hybridization's of frames a and b serve to show that the same cells express T2R and gustducin. And the last two frames show that T1R and T2R are expressed in different cells. Therefore, we may conclude that T2R is both unique from T1R, and therefore a novel a receptor, as well as a possible gustducin linked receptor. The idea that it is only possible that gustducin and T2R are linked is critical. I do not believe they proved that they are necessarily linked, however, it is a possibility. On the other hand, I feel that the real proof is that T2R is a new an unique receptor type, different from T1R. Not only do they differ in sequence and transmembrane areas, but most importantly in the cells which they are expressed. This knowledge help further understand taste biology, and particularly bitter taste perception. Future studies could focus on two aspects: proving the coupling of T2R and gustducin, and in vivo studies. They paper showed that gustducin is necessary for T2R expression, but it did not show positive proof of linkage or coupling. One study that's implications may be used involves the use of alpha-gustducin promoters (Wong). The promoters studies show the development and utility of gustducin in bitter taste perception. A similar study may be conducted evaluating T2R. Another area of potential research is not only using in vitro and in situ experiments, but also in vivo. An example of this type of research is being conducted by Dr. Robert Margolskee. These experiments use green fluorescent proteins to look at gene expression in taste cells. By looking at actual in vivo cells we might understand more real application of taste perception.
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