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Taste receptor

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Taste receptor
Taste receptors of the tongue are present in the taste buds of papillae.
Identifiers
FMA84662
Anatomical terminology

A taste receptor is a type of receptor which facilitates the sensation of taste. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity and other locations. Molecules which give a sensation of taste are considered "sapid".[1]

Taste receptors are divided into two families:

Combinations of these receptors in dimers or other complexes contributes to different perceptions of taste.

Visual, olfactive, “sapictive” (the perception of tastes), trigeminal (hot, cool), mechanical, all contribute to the perception of taste. Of these, transient receptor potential cation channel subfamily V member 1 (TRPV1) vanilloid receptors are responsible for the perception of heat from some molecules such as capsaicin, and a CMR1 receptor is responsible for the perception of cold from molecules such as menthol, eucalyptol, and icilin.[1]

Tissue distribution

The gustatory system consists of taste receptor cells in taste buds. Taste buds, in turn, are contained in structures called papillae. There are three types of papillae involved in taste: fungiform papillae, foliate papillae, and circumvallate papillae. (The fourth type - filiform papillae do not contain taste buds). Beyond the papillae, taste receptors are also in the palate and early parts of the digestive system like the larynx and upper esophagus. There are three cranial nerves that innervate the tongue; the vagus nerve, glossopharyngeal nerve, and the facial nerve. The glossopharyngeal nerve and the chorda tympani branch of the facial nerve innervate the TAS1R and TAS2R taste receptors.

In 2010, researchers found bitter receptors in lung tissue, which cause airways to relax when a bitter substance is encountered. They believe this mechanism is evolutionarily adaptive because it helps clear lung infections, but could also be exploited to treat asthma and chronic obstructive pulmonary disease.[4]

Function

Taste helps to identify toxins and maintain nutrition. Five basic tastes are recognized today: salty, sweet, bitter, sour, and umami. Salty and sour taste sensations are both detected through ion channels. Sweet, bitter, and umami tastes, however, are detected by way of G protein-coupled taste receptors.[5]

In addition, some agents can function as taste modifiers, as miraculin or curculin for sweet or sterubin to mask bitter.

Mechanism of action

The standard bitter, sweet, or umami taste receptor is a G protein-coupled receptor with seven transmembrane domains. Ligand binding at the taste receptors activate second messenger cascades to depolarize the taste cell. Gustducin is the most common taste Gα subunit, having a major role in TAS2R bitter taste reception. Gustducin is a homologue for transducin, a G-protein involved in vision transduction.[6] Additionally, taste receptors share the use of the TRPM5 ion channel, as well as a phospholipase PLCβ2.[7]

Savory or Glutamates

The TAS1R1+TAS1R3 heterodimer receptor functions as the savory receptor, responding to L-amino acid binding, especially L-glutamate.[2] The umami taste is most frequently associated with the food additive monosodium glutamate (MSG) and can be enhanced through the binding of inosine monophosphate (IMP) and guanosine monophosphate (GMP) molecules.[8][9] TAS1R1+3 expressing cells are found mostly in the fungiform papillae at the tip and edges of the tongue and palate taste receptor cells in the roof of the mouth.[2] These cells are shown to synapse upon the chorda tympani nerves to send their signals to the brain, although some activation of the glossopharyngeal nerve has been found.[8][10]

Sweet

The diagram above depicts the signal transduction pathway of the sweet taste. Object A is a taste bud, object B is one taste cell of the taste bud, and object C is the neuron attached to the taste cell. I. Part I shows the reception of a molecule. 1. Sugar, the first messenger, binds to a protein receptor on the cell membrane. II. Part II shows the transduction of the relay molecules. 2. G Protein-coupled receptors, second messengers, are activated. 3. G Proteins activate adenylate cyclase, an enzyme, which increases the cAMP concentration. Depolarization occurs. 4. The energy, from step 3, is given to activate the K+, potassium, protein channels.III. Part III shows the response of the taste cell. 5. Ca+, calcium, protein channels is activated.6. The increased Ca+ concentration activates neurotransmitter vesicles. 7. The neuron connected to the taste bud is stimulated by the neurotransmitters.

The TAS1R2+TAS1R3 heterodimer receptor functions as the sweet receptor by binding to a wide variety of sugars and sugar substitutes.[2][11] TAS1R2+3 expressing cells are found in circumvallate papillae and foliate papillae near the back of the tongue and palate taste receptor cells in the roof of the mouth.[2] These cells are shown to synapse upon the chorda tympani and glossopharyngeal nerves to send their signals to the brain.[5][10] The TAS1R3 homodimer also functions as a sweet receptor in much the same way as TAS1R2+3 but has decreased sensitivity to sweet substances. Natural sugars are more easily detected by the TAS1R3 receptor than sugar substitutes. This may help explain why sugar and artificial sweeteners have different tastes.[12]

Bitter

The TAS2R proteins function as bitter taste receptors.[13] There are 43 human TAS2R genes, each of which (excluding the five pseudogenes) lacks introns and codes for a GPCR protein.[5] These proteins, as opposed to TAS1R proteins, have short extracellular domains and are located in circumvallate papillae, palate, foliate papillae, and epiglottis taste buds, with reduced expression in fungiform papillae.[3][5] Though it is certain that multiple TAS2Rs are expressed in one taste receptor cell, it is still debated whether mammals can distinguish between the tastes of different bitter ligands.[3][5] Some overlap must occur, however, as there are far more bitter compounds than there are TAS2R genes. Common bitter ligands include cycloheximide, denatonium, PROP (6-n-propyl-2-thiouracil), PTC (phenylthiocarbamide), and β-glucopyranosides.[5]

Signal transduction of bitter stimuli is accomplished via the α-subunit of gustducin. This G protein subunit activates a taste phosphodiesterase and decreases cyclic nucleotide levels. Further steps in the transduction pathway are still unknown. The βγ-subunit of gustducin also mediates taste by activating IP3 (inositol triphosphate) and DAG (diglyceride). These second messengers may open gated ion channels or may cause release of internal calcium.[14] Though all TAS2Rs are located in gustducin-containing cells, knockout of gustducin does not completely abolish sensitivity to bitter compounds, suggesting a redundant mechanism for bitter tasting[7] (unsurprising given that a bitter taste generally signals the presence of a toxin).[7] One proposed mechanism for gustducin-independent bitter tasting is via ion channel interaction by specific bitter ligands, similar to the ion channel interaction which occurs in the tasting of sour and salty stimuli.[5]

One of the best-researched TAS2R proteins is TAS2R38, which contributes to the tasting of both PROP and PTC. It is also the only taste receptor whose polymorphisms are shown to be responsible for differences in taste perception. Current studies are focused on determining other such taste phenotype-determining polymorphisms.[5]

The diagram depicted above shows the signal transduction pathway of the bitter taste. Bitter taste has many different receptors and signal transduction pathways. Bitter indicates poison to animals. It is most similar to sweet. Object A is a taste bud, object B is one taste cell, and object C is a neuron attached to object B. I. Part I is the reception of a molecule.1. A bitter substance such as quinine, is consumed and binds to G Protein-coupled receptors.II. Part II is the transduction pathway 2. Gustducin, a G protein second messenger, is activated. 3. Phosphodiesterase, an enzyme, is then activated. 4. Cyclic nucleotide, cNMP, is used, lowering the concentration 5. Channels such as the K+, potassium, channels, close.III. Part III is the response of the taste cell. 6. This leads to increased levels of Ca+. 7. The neurotransmitters are activated. 8. The signal is sent to the neuron.

Sour

Historically it was thought that the sour taste was produced solely when free hydrogen ions (H+) directly depolarised taste receptors. However, specific receptors for sour taste with other methods of action are now being proposed. HCN1 and HCN4 (HCN channels) were two such proposals; both of these receptors are cyclic nucleotide-gated channels. The two ion channels suggested to contribute to sour taste are ACCN1 and TASK-1.

The diagram depicts the signal transduction pathway of the sour or salty taste. Object A is a taste bud, object B is a taste receptor cell within object A, and object C is the neuron attached to object B. I. Part I is the reception of hydrogen ions or sodium ions. 1. If the taste is sour, H+ ions, from an acidic substances, pass through their specific ion channel. Some can go through the Na+ channels. If the taste is salty Na+, sodium, molecules pass through the Na+ channels. Depolarization takes place II. Part II is the transduction pathway of the relay molecules.2. Cation, such as K+, channels are opened. III. Part III is the response of the cell. 3. An influx of Ca+ ions is activated.4. The Ca+ activates neurotransmitters. 5. A signal is sent to the neuron attached to the taste bud.

Salt

Various receptors have also been proposed for salty tastes, along with the possible taste detection of lipids, complex carbohydrates, and water. Evidence for these receptors is, however, shaky at best, and is often unconvincing in mammal studies. For example, the proposed ENaC receptor for sodium detection can only be shown to contribute to sodium taste in Drosophilia.[5]

Carbonation

An enzyme connected to the sour receptor transmits information about carbonated water.[15]

Fat

A possible taste receptor for fat, CD36, has been identified.[16] CD36 has been localized to the circumvallate and foliate papillae, which are present in taste buds,[17] and research has shown that the CD36 receptor binds long chain fatty acids.[18] Differences in the amount of CD36 expression in human subjects was associated with their ability to taste fats,[19] creating a case for the receptor's relationship to fat tasting. Further research into the CD36 receptor could be useful in determining the existence of a true fat-tasting receptor.

GPR120 and GPR40 have been implicated to respond to oral fat,[20] and their absence leads to reduced fat preference and reduced neuronal response to orally administered fatty acids.[21]

TRPM5 has been shown to be involved in oral fat response and identified as a possible oral fat receptor, but recent evidence presents it as primarily a downstream actor.[22][23]

Types

Human bitter taste receptor genes are named TAS2R1 to TAS2R64, with many gaps due to non-existent genes, pseudogenes or proposed genes that have not been annotated to the most recent human genome assembly. Many bitter taste receptor genes also have confusing synonym names with several different gene names referring to the same gene. See table below for full list of human bitter taste receptor genes:

Class Gene Synonyms Aliases Locus Description
type 1
(sweet) 		TAS1R1 GPR70 1p36.23
TAS1R2 GPR71 1p36.23
TAS1R3 1p36
type 2
(bitter) 		TAS2R1 5p15
TAS2R2 7p21.3 pseudogene
TAS2R3 7q31.3-q32
TAS2R4 7q31.3-q32
TAS2R5 7q31.3-q32
TAS2R6 7 not annotated in human genome assembly
TAS2R7 12p13
TAS2R8 12p13
TAS2R9 12p13
TAS2R10 12p13
TAS2R11 absent in humans
TAS2R12 TAS2R26 12p13.2 pseudogene
TAS2R13 12p13
TAS2R14 12p13
TAS2R15 12p13.2 pseudogene
TAS2R16 7q31.1-q31.3
TAS2R17 absent in humans
TAS2R18 12p13.2 pseudogene
TAS2R19 TAS2R23, TAS2R48 12p13.2
TAS2R20 TAS2R49 12p13.2
TAS2R21 absent in humans
TAS2R22 12 not annotated in human genome assembly
TAS2R24 absent in humans
TAS2R25 absent in humans
TAS2R27 absent in humans
TAS2R28 absent in humans
TAS2R29 absent in humans
TAS2R30 TAS2R47 12p13.2
TAS2R31 TAS2R44 12p13.2
TAS2R32 absent in humans
TAS2R33 12 not annotated in human genome assembly
TAS2R34 absent in humans
TAS2R35 absent in humans
TAS2R36 12 not annotated in human genome assembly
TAS2R37 12 not annotated in human genome assembly
TAS2R38 7q34
TAS2R39 7q34
TAS2R40 GPR60 7q34
TAS2R41 7q34
TAS2R42 12p13
TAS2R43 12p13.2
TAS2R45 GPR59 12
TAS2R46 12p13.2
TAS2R50 TAS2R51 12p13.2
TAS2R52 absent in humans
TAS2R53 absent in humans
TAS2R54 absent in humans
TAS2R55 absent in humans
TAS2R56 absent in humans
TAS2R57 absent in humans
TAS2R58 absent in humans
TAS2R59 absent in humans
TAS2R60 7
TAS2R62P 7q34 pseudogene
TAS2R63P 12p13.2 pseudogene
TAS2R64P 12p13.2 pseudogene

References

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  22. ^ Mattes RD (Sep 2011). "Accumulating evidence supports a taste component for free fatty acids in humans". Physiology & Behavior. 104 (4): 624–31. doi:10.1016/j.physbeh.2011.05.002. PMC 3139746. PMID 21557960.
  23. ^ Liu P, Shah BP, Croasdell S, Gilbertson TA (Jun 2011). "Transient receptor potential channel type M5 is essential for fat taste". The Journal of Neuroscience. 31 (23): 8634–42. doi:10.1523/JNEUROSCI.6273-10.2011. PMC 3125678. PMID 21653867.
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