The molecular systematics behind taste.

Taste is one of the bodys sensory systems. Taste is divided into five primary sensations: bitter,sweet, sour, salty and umami (the taste of glutamate). Taste evolved to help separate nutrientfoods (sweet, salty and umami) from potential harmful ones (sour and bitter). The systemsresponding to the tastants (the molecules sensed by taste) are quite different for the differenttastes. The mechanisms behind sour and salty tastes are the simplest ones. Their tastes aredetected by the passage of their ions, hydrogen ions and sodium ions respectively, through theion channels in taste bud cells on the tongue. The tastes of sweet, bitter and umami are allperceived by receptors, but the mechanisms behind them are diverse. The molecularmechanisms behind the taste receptions will be discussed in detail in the following chapters.Saliva has also a role in the process of taste reception in the oral cavity. It dissolves tastesubstances and helps transport them to the TRCs (taste receptor cells). It can also constantlystimulate the TRCs resulting in an alteration of taste sensitivity.

Located on the tongue and the palate in theoral cavity are the taste papillae thatinitiates the sensation of taste. Eachpapillae contains one or more taste buds.They are named after their onion-shapedstructures that are embedded in theepithelium of the tongue. Each taste budcontains ~100 of cells that includes TRCs.The TRCs are specialized epithelial cellsthat have neuronal properties. Theyperceive the tastants and forms synapseswith afferent neurons which in turn sendssignals to the brainstem and the thalamus.Taste buds are found in three places on thetongue: in fungiform at the front, in foliateat the sides and in vallate at the rear of thetongue. In figure 1 are the locations of thepapillae and taste buds represented. Tastebuds are also found in the palate, uvula,epiglottis, pharynx, larynx and esophagus.

Figure 1. The locations of papillae andtaste buds. A) The placement of tastepapillae on the tongue. B) The placementof taste buds in the papillae. C) The looksof a taste bud.

Bitter transduction and its pathways.

Scientists have found a family of GPCR (G protein coupled receptors) genes that code for thereceptors of bitter taste. They are called T2Rs (also called TRBs). They display 30 – 70 %identity within the gene family. The receptors have the greatest conservation in theircytoplasmic loops and in their adjacent transmembrane segments (probable sites for G proteininteraction). Their biggest diversity are found in their extracellular regions (possible regionsof ligand binding). Half of all TRCs contains a transient receptor channel, TRMP5, which isrequired for sweet, bitter and umami taste and is almost always co-expressed with PLC_2(phosholipase C, _2 subunit). Scientists have also found a _-transducin-like G protein _-subunit named _-Gustducin which is selectively expressed in ~25 – 30 % of TRCs. Tests haveshown that it is involved in bitter taste transduction. Also the __-subunits (G_3 and G_13) ofgustducin are involved in bitter taste transduction. They produce an increase in taste tissuelevels of inositol triphophate (IP3) (cellular second messenger code) and diacylglycerol(DAG) (cellular second messenger code). Other pathways for bitter transduction have alsobeen suggested because knocking out _-gustducin in mice have only reduced the response tobitter taste. The other candidates suggested are G_i-3, G_14, G_15, G_q, G_s and _-transducinbecause they are all prescent in TRCs. Not all bitter-responsive cells express gustducinhowever, which indicates that T2Rs may not be the only bitter receptors in the oral cavity.

There are two known pathways for bitter transduction. Gustducin heterotrimers that have beenactivated by T2R/TRB receptors produces two responses in TRCs. One is a decrease incNMPs (taste tissue cyclic nucleotide) (cellular second messenger code) via _-gustducin. Thesubsequent steps in this pathway are still unknown. The decreased cNMPs can act on proteinkinases, which in turn can regulate the activity of the TRC ion channel, or the cNMP levelscan directly control the activity of cNMP-gated and cNMP-inhibited ion channels existing inthe TRCs. The second pathway is that the gustducin heterotrimer produces a rise in IP3/DAGvia __-gustducin. The subsequent steps in this reaction are probably the activation of IP3

receptors and the release of Ca2+ from internal stores followed by neurotransmitter release. Infigure 2 are the two possible pathways drawn.

Sweet transduction and pathways.

Studies have shown that the GPCR gene responsibe for sweet receptors is T1R3. T1R3 is~30% related to T1R1 and T1R2. T1R3 is exclusively present in TRCs within fungiform,foliate and circumvallate papillae. T1R3 can alone response to sweet tastes if theconcentrations of sweet compounds are very high. But usually T1R3 form heteromers withT1R2 that responses to sweet taste. This heterotrimer is also responsive to artificialsweeteners.The G protein _-subunit, _-gustducin, is also used in sweet transduction as areTRMP5 and PLC_2. Current models tells that adenylyl cyclase (AC)-generated cAMP andPLC_2-generated IP3 are second messengers in sweet transduction.

As in bitter transduction there are two possible pathways in sweet transduction. The first is aGPCR-Gs-cAMP pathway. Sucrose and other sugars activate Gs via one or more coupledGPCRs. The receptor-activated G_s then activates adenylyl cyclase to generate cAMP. Thegenerated cAMPmay then act directly to cause cation influx through cNMP-gated channels oract indirectly to activate protein kinase A, which phosphorylates a basolateral K+ channel.This leads to the closure of the channel, depolarization of the taste cell, voltage-dependent

Ca2+ influx and neurotransmitter realease to the brainstem. The second pathway is GPCR-Gq/G__-IP3. Artificial sweeteners bind to to and activate one or more GPCRs coupled toPLC_2 by either the _-subunit of Gq or by G__ subunits. Activated G_q or released G__ thenactivates PLC_2 to generate IP3 and DAG. The IP3 and DAG elicit Ca2+ release from theinternal stores, which leads to depolarization of the TRC and neurotransmitter release. Thetwo possible pathways are shown in figure 2.

Figure 2. The possible reaction ways.A) Salty transduction B) Sour transduction C) Umami transduction D) Sweet transductionE) Bitter transduction

Taste of salt and sour.

Those could be say, to belong to the same category, and are detected primarily by the passageof ions through channels expressed on the surface of cells in the tongue.

Na+ elicits a prototypic taste sensation in humans, called salty taste. We’ll first look onwhat’s happening when salt comes on our tongue.This discovery of transport pathways for Na+ was made in the early eighties.

Many organism have the ability to detect Na+ in food. The cells who can do this, are calledtaste receptor cells (TRC)On the tongue, taste buds are housed in particular formations called taste papillae

The function of TRCs is to transform chemical signals into series of action potentials in thenerve fibers towards the brain. Ion channels of the epithelial Na+ channel super family are key molecules in the detection ofNa+ and H+ by vertebrate TRCs

Several types of Potassium channels could be found in vertebrate TRCs, these channelsmakes as example, the depolarization phase of action potentials.Stimuli to the channels are made with different types of ions, including H+, K+, Ca2+ andquinine.These stimuli, either "go through" the channels (K+), or block them (H+. Ca2+, quinine) andgive an inhibit of the voltage-sensitive K+ channels. In any case, the stimulus action brings about a membrane depolarization, considered to be anreceptor potential, and play a key role in the transduction of taste stimuli into receptorpotentials

Here are some examples of taste stimuli:Na+ - SaltyK+ - Salty-bitterH+ - Sour.

The voltage that builds up in the channels varies from -47 mV to -10 mV.These channels open up for a membrane depolarization, they are also active at rest, andcontribute to the detection of taste stimuli.Regulation are made by ATP, who reversibly block channel activity, and by Ca2+, whoincrease it.

Amiloride-sensitive sodium channel (ASSCs) is one of the most important channels, the namecomes from the fact that they are sensitive to amiloride. But nowadays they’re calledepithelial sodium channels (ENaC).Taste ENaC are similar to those found in absorbing epithelial cells, such as those in kidney,lung and colon, where they mediate transport of sodium ions.It comprises four subunits that are homologous in some way, either identical or distinct.One subunit is about 500- 1000 amino acids and includes two presumed membrane-spanninghelices as well as a large extracellular domain in between them. Sodium ions passing throughtheses channels produce a significant trans-membrane current.

Functional properties and regulation of the amiloride-sensitive channel that works astaste receptor for Na+ and H+.

Amiloride and benzamil are specific inhibitors of ENaC (Ki < 1 microM) and blocks thechannel reversibly from the outside.The channel also exhibits sodium self-inhibition at the extra cellular aspect.

Taste ENaC are also under hormonal control so is the regulation of salt and water balance.Vasopressin regulates channel activity likely via the second messsenger cAMP.Aldosterone is thought to increase the total amiloride-sensitive currents through ENaCregulation.

Stimulis translocation through open channels causes membrane depolarization. In case of protons, a reduction in the intracellular pH is expected to occur.

Sodium detection in rodents, and possibly in other vertebrates is thought to be mediated bythe inflow of Na+ through ENaC at the apical membrane, and the outflow of Na+ via theNa+/K+ pump on the basolateral membrane. Due to this polarization of the molecularcomponents, either muxosal amiloride or serosal ouabain can affect the trans-epithelialcurrent and, therefore, transduction. In figure 2 are the potential pathways listed for both saltyand sour tastes.

The fifth taste??

In many years it has been said that the human tongue can detect only four basic tastes: sweet,sour, bitter and salty, and that all tastes are combinations of these. In recent years some workers have added a fifth taste, umami, to the other four, the taste ofGlutamate.Both the word and the concept are Japanese, and in Japan are of some antiquity. Umami ishard to translate, but it means almost deliciousness, and meaty. It’s sometimes associated with a feeling of perfect quality in a taste, or of some specialemotional circumstance in which a taste is experienced.

Unami are present in protein rich foods, and can be detected of adults by an concentration ofglutamate at 1 mM.

These amino acids are detected by a specialized glutamate receptor and the stimuli areprimarily from L-glutamat. The unami receptors is a variant of a brain glutamate receptor.A substantial part of the high-affinity glutamate-binding domain is missing in the formexpressed in the tongue (the yellow). And the binding sites for L-glutamat could be increasedby GMP, this could verify that the receptor is allosteric. In figure 2 is the possible pathway forumami listed.

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