Fourth taste receptor identified
A team led by Charles Zuker, of the Howard Hughes Medical Institute, has identified a receptor which mediates sour taste and is expressed in a unique cell type in the taste buds. The work is reported in today’s issue of Nature; another group reports the same findings in the Proceedings of the National Academy of Sciences (full access).
Humans have four basic taste senses: sweet, salty, bitter, sour. A fifth, umami, is the taste of monosodium glutamate. Receptors for bitter tastes are the most sensitive, as poisonous chemicals are bitter-tasting.
Taste receptors are found in the taste buds on the tongue. Each taste bud consists of 30-100 taste receptor cells, which are elongated neuroepithelial cells embedded in specialized regions of the surrounding epithelium called papillae. There are four types of papillae, characterised by their shapes: fungiform (mushroom-shaped), filiform (thread-shaped), foliate (leaf-shaped) and circumvallate (circular). All but the filiform papillae have taste buds on their surface.
Within the taste bud , taste receptor cells project microvilli to the taste pore, where interactions between taste receptors and tastant chemicals takes place. Chemicals must first dissolve in a film of liquid coating taste receptor cell membranes for tasting to occur.
Zucker’s team, who previously identified the sweet, bitter and umami receptors, used a combination of bioinformatics, molecular biology to identify, isolate and clone the sour receptor. Because receptors are embedded in cell membranes, a mouse genome database was scanned to find open reading frames containing one or more putative transmembrane domains; of the 10,000 genes found, 900 are expressed in taste receptor cells. In situ hybridization was then used to compare the expression patterns of these 900 genes with those of known taste receptors.
By this process of elimination, a single receptor, PKD2L1, which is expressed in a unique subset of taste receptor cells, was identified.
PKD2L1 (for polycystic-kidney-disease-like ion channel) is a proton-gated ion channel expressed in the taste pores of a subset of taste receptor cells in circumvallate and foliate taste buds (left). The sequence of PKD2L1 is similar to that of another ion channel, PKD2, which is mutated in autosomal-dominant polycystic kidney disease.
A strain of transgenic mice expressing diptheria toxin in PKD2L1-positive cells was engineered, so that the cells which would normally express the receptor were killed. Microelectrodes were then used to record electrical activity in the nerves innervating the taste receptors cells (the facial and glossopharyngeal nerves, which are the seventh and ninth cranial nerves, respectively). This showed that mice lacking PKD2L1 did not avoid acidic foods like normal animals, suggesting that they had lost the ability to taste sour chemicals. In contrast, the response of mice with an ablated PKD2L1 gene to other tastes did not differ from normal mice.
These findings also support the labelled-line theory, according to which, different taste qualities are encoded by specific groups of neurons.
PKD2L1 was also shown to be expressed in a discrete population of cells surrounding the central canal of the spinal cord. This second population of PKD2L1-positive cells are found along the entire length of the spinal cord, and extend ciliated processes into the central canal.
The authors propose that PKD2L1 may play a role in homeostasis by monitoring carbon dioxide levels in the cerebrospinal fluid. This was confirmed by the creation of a second transgenic mouse strain expressing green fluorescent protein in PKD2L1-positive cells. Patch-clamp recordings from these cells showed that their responses to acid stimulation were very different to those of taste receptor cells. Whereas the PKD2L1-expressing cells in the taste buds are responsive to a pH range of 2-5, those in the spinal cord were far more sensitive, responding to very small changes in proton concentration.
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Besides being seen on taste buds, PKD2L1 were also detected in spinal tissues. Prof. Isidro T. Savillo, President of ISMCBBPR further stated that, “There may be an evolutionary implication because taste receptors were reported to exist in unusual body parts of some fishes like upper lobe of a caudal fin, skin in addition to those present in fish lips’ and palate. In addition, their presence in the spinal tissues may also support the important role of pH in the overall physiology of mammals.”
The Molecule of the Year 2007 is the PKD2L1. Through the discretion of the President of ISMCBBPR (International Society for Molecular and Cell Biology and Biotechnology Protocols and Researches), the Molecule of the Year 2007 is the PKD2L1. PKD2L1 is the fourth taste receptor identified which detects sour taste and was found expressed by particular cells in taste buds. Moreover the techniques (e.g. bioinformatics, etc.) and molecular protocols which were used to scout and subsequently locate these ionic channels were profound and exceedingly exciting. In addition, Charles Zuker and his team were also known to identify other mammalian taste receptors (e.g. for sweet, bitter and umami). “The honor bestowed upon PKD2L1 embodies all the other taste receptors they discovered and will uncover in the near future,” says Prof. Isidro T. Savillo, President of ISMCBBPR. Congratulations to Dr. Charles Zuker and his Team!! Please visit http://www.webspawner.com/users/moloftheyear2007/index.html
Source:ScientistSolutions