Pathway: Sensory perception of sweet, bitter, and umami (glutamate) taste
Reactions in pathway: Sensory perception of sweet, bitter, and umami (glutamate) taste :
Sensory perception of sweet, bitter, and umami (glutamate) taste
Taste receptors for bitter compounds, sweet compounds, and umami compounds (L-glutamate in humans, several amino acids in mice) are G protein-coupled receptors located in type II taste bud cells that signal through a common downstream pathway (reviewed in Margolskee 2002, Kinnamon 2009, Kurihara 2015, Roper and Chauhari et al. 2017, Kinnamon and Finger 2019, Servant et al. 2020). Umami ("savoury", L-glutamate) taste receptors are heterodimers of the plasma membrane proteins TAS1R1 and TAS1R3. TAS1R1:TAS1R3 heterodimers also bind 5' nucleotides such as 5' IMP which synergistically augment umami taste. The glutamate receptors GRM1 (mGluR1) and GRM4 (mGluR4) act in an alternative pathway for sensing glutamate in taste cells (reviewed in Chaudhari et al. 2009). Sweet taste receptors are heterodimers of the plasma membrane proteins TAS1R2 and TAS1R3 (reviewed in Yang et al. 2021). The glucose transporters SGLT1 and GLUT4 are expressed in type II taste cells and may provide an alternative pathway for sensing glucose (reviewed in von Molitor et al. 2020). Bitter receptors are a large family of monomeric plasma membrane proteins, the TAS2R proteins.
TAS1R-containing sweet and umami receptors and TAS2R bitter receptors are each physically associated with a particular heterotrimeric G protein complex, the gustducin complex, containing GNAT3 (gustducin), GNB1 or GNB3, and GNG13. Upon binding an agonist ligand, the receptor activates the alpha subunit, GNAT3, to exchange GDP for GTP, which results in a conformational change in GNAT3 that causes the receptor-gustducin complex to dissociate, yielding GNAT3:GTP, GNB1,3:GNG13, and the receptor:ligand. The GNB1,3:GNG13 complex binds and activates Phospholipase C beta-2 (PLCB2), which then hydrolyzes phosphoinositol 4,5-bisphosphate (PI(4,5)P2) to yield diacylglycerol and inositol 1,4,5-trisphosphate (I(1,4,5)P3). I(1,4,5)P3 binds and activates the calcium channel IP3-gated Ca-channel type 3 (ITPR3) and ITPR3 then releases calcium ions from the endoplasmic reticulum into the cytosol. The increased cytosolic calcium activates the TRPM5 cation channels, which then transport sodium ions along the concentration gradient from the extracellular region to the cytosol (reviewed in Aroke et al. 2020). The depolarization activates SCN2A, SCN3A, and SCN9A channels, which transport further sodium ions from the extracellular region to the cytosol. The depolarization of the plasma membrane opens CALHM1:CALHM3 channels, which transport ATP, a neurotransmitter in the olfactory system, from the cytosol to the extracellular region.
Taste receptors were initially discovered in taste buds of the tongue and have now been found in several other tissues including nasal epithelium (Barnham et al. 2015, inferred from rodent homologs in Tizzano et al. 2011), the respiratory system, pancreatic islet cells, sperm (Governini et al. 2020), leukocytes (Malki et al. 2015), and enteroendocrine cells of the gut (inferred from rat and mouse homologs in Wu et al. 2002).
TAS1R-containing sweet and umami receptors and TAS2R bitter receptors are each physically associated with a particular heterotrimeric G protein complex, the gustducin complex, containing GNAT3 (gustducin), GNB1 or GNB3, and GNG13. Upon binding an agonist ligand, the receptor activates the alpha subunit, GNAT3, to exchange GDP for GTP, which results in a conformational change in GNAT3 that causes the receptor-gustducin complex to dissociate, yielding GNAT3:GTP, GNB1,3:GNG13, and the receptor:ligand. The GNB1,3:GNG13 complex binds and activates Phospholipase C beta-2 (PLCB2), which then hydrolyzes phosphoinositol 4,5-bisphosphate (PI(4,5)P2) to yield diacylglycerol and inositol 1,4,5-trisphosphate (I(1,4,5)P3). I(1,4,5)P3 binds and activates the calcium channel IP3-gated Ca-channel type 3 (ITPR3) and ITPR3 then releases calcium ions from the endoplasmic reticulum into the cytosol. The increased cytosolic calcium activates the TRPM5 cation channels, which then transport sodium ions along the concentration gradient from the extracellular region to the cytosol (reviewed in Aroke et al. 2020). The depolarization activates SCN2A, SCN3A, and SCN9A channels, which transport further sodium ions from the extracellular region to the cytosol. The depolarization of the plasma membrane opens CALHM1:CALHM3 channels, which transport ATP, a neurotransmitter in the olfactory system, from the cytosol to the extracellular region.
Taste receptors were initially discovered in taste buds of the tongue and have now been found in several other tissues including nasal epithelium (Barnham et al. 2015, inferred from rodent homologs in Tizzano et al. 2011), the respiratory system, pancreatic islet cells, sperm (Governini et al. 2020), leukocytes (Malki et al. 2015), and enteroendocrine cells of the gut (inferred from rat and mouse homologs in Wu et al. 2002).
Taste buds contain at least 3 types of cells: type I cells appear to have a support (glial-like) function; type II cells are responsible for tasting sweet compounds, bitter compounds, and umami (savoury, amino acid) compounds; and type III cells are responsible for tasting sour (acidic) compounds (reviewed in Liman et al. 2014, Roper and Chaudhari 2017, Kinnamon and Finger 2019, Taruno et al. 2021). Recently identified sodium sensing cells expressing the epithelial sodium channel (ENaC) and POU2F3 are thought to be responsible for tasting low concentrations of salt and may be a subset of type II cells or a novel type of taste cell (Chandrashekar et al. 2010, reviewed in Taruno et al. 2021). High concentrations of salt appear to be detected by both type II and type III cells.
Receptors for sweet compounds, bitter compounds, and umami compounds contain an intracellular domain, transmembrane domains, and an extracellular domain that binds the ligand. The extracellular domains of receptors for sweet and umami ligands have a distinctive "venus flytrap"-shaped domain. Upon binding ligand, sweet taste receptors (TAS1R2:TAS1R3 heterodimers), bitter taste receptors (TAS2R class receptors), and umami receptors (TAS1R1:TAS1R3 heterodimers) then signal through a common downstream pathway: the receptor-ligand complex activates an associated heterotrimeric G protein complex (GNAT3:GNB1 or GNB3:GNG13) to exchange GDP for GTP, the heterotrimeric G protein complex dissociates and the resulting GNB1,3:GNG13 complex activates Phospholipase C beta-2 (PLCB2) which hydrolyzes phosphoinositol 4,5-bisphosphate (PI(4,5)P2) to yield inositol 1,4,5-trisphosphate (I(1,4,5)P3) and diacylglycerol (DAG). I(1,4,5)P3 binds and activates ITPR3 to release calcium ions from the endoplasmic reticulum into the cytosol. Cytosolic Ca2+ causes TRPM5 sodium channels to open and depolarize the cell. SCN2A, SCN3A, and SCN9A sodium channels also appear to augment the depolarization. Depolarization causes opening of CALHM1:CALHM3 channels which transport ATP from the cytosol to the extracellular region. ATP then acts as a neurotransmitter in the taste sensing system.
Alternative pathways exist for sensing sugars and glutamate, as evidenced by residual signaling activity in the absence of TAS1R1 or TAS1R3. Glutamate is sensed by the glutamate receptors GRM1 (mGluR1) and GRM4 (mGluR4) expressed in type II taste cells. GRM1 and GRM4 activate calcium channels by an incompletely characterized mechanism that probably involves heterotrimeric G proteins. Glucose may be sensed by a pathway comprising transport into type II taste cells via the glucose transporters SGLT1 and GLUT4, generation of ATP, and inhibition of KATP potassium channels by ATP.
Protons (H+ ions) from acidic compounds translocate from the extracellular region to the cytosol of type III taste cells through the OTOP1 channel. Weak acids such as acetic acid and citric acid are also able to enter type III cells by diffusing through the membrane in their protonated, uncharged forms, Once in the cytosol, the H+ ions inhibit KCNJ2 inwardly rectifying potassium channels, depolarizing the cell. The H+ ions may also open unidentified sodium channels to further depolarize the cell. Depolarization causes exocytosis of the neurotransmitters serotonin (5-HT) and gamma-aminobutyric acid (GABA).
Low concentrations of salt appear to be sensed in specific salt-sensing cells that may be a subset of type II cells. Low concentrations of salt are believed to enter the cell through an epithelial sodium channel (ENaC, SCNN) and the ability to taste low concentrations of salt is dependent on the SCNN1A pore-containing subunit of the SCNN complex in mice. Human taste cells express both SCNN1A and SCNN1D pore-containing subunits. The composition of other subunits of the complex is less certain. The transport of sodium ions (Na+) into the cells depolarizes the plasma membrane and eventually leads to opening of CALHM1:CALHM3 channels which transport ATP from the cytosol to the extracellular region.
Receptors for sweet compounds, bitter compounds, and umami compounds contain an intracellular domain, transmembrane domains, and an extracellular domain that binds the ligand. The extracellular domains of receptors for sweet and umami ligands have a distinctive "venus flytrap"-shaped domain. Upon binding ligand, sweet taste receptors (TAS1R2:TAS1R3 heterodimers), bitter taste receptors (TAS2R class receptors), and umami receptors (TAS1R1:TAS1R3 heterodimers) then signal through a common downstream pathway: the receptor-ligand complex activates an associated heterotrimeric G protein complex (GNAT3:GNB1 or GNB3:GNG13) to exchange GDP for GTP, the heterotrimeric G protein complex dissociates and the resulting GNB1,3:GNG13 complex activates Phospholipase C beta-2 (PLCB2) which hydrolyzes phosphoinositol 4,5-bisphosphate (PI(4,5)P2) to yield inositol 1,4,5-trisphosphate (I(1,4,5)P3) and diacylglycerol (DAG). I(1,4,5)P3 binds and activates ITPR3 to release calcium ions from the endoplasmic reticulum into the cytosol. Cytosolic Ca2+ causes TRPM5 sodium channels to open and depolarize the cell. SCN2A, SCN3A, and SCN9A sodium channels also appear to augment the depolarization. Depolarization causes opening of CALHM1:CALHM3 channels which transport ATP from the cytosol to the extracellular region. ATP then acts as a neurotransmitter in the taste sensing system.
Alternative pathways exist for sensing sugars and glutamate, as evidenced by residual signaling activity in the absence of TAS1R1 or TAS1R3. Glutamate is sensed by the glutamate receptors GRM1 (mGluR1) and GRM4 (mGluR4) expressed in type II taste cells. GRM1 and GRM4 activate calcium channels by an incompletely characterized mechanism that probably involves heterotrimeric G proteins. Glucose may be sensed by a pathway comprising transport into type II taste cells via the glucose transporters SGLT1 and GLUT4, generation of ATP, and inhibition of KATP potassium channels by ATP.
Protons (H+ ions) from acidic compounds translocate from the extracellular region to the cytosol of type III taste cells through the OTOP1 channel. Weak acids such as acetic acid and citric acid are also able to enter type III cells by diffusing through the membrane in their protonated, uncharged forms, Once in the cytosol, the H+ ions inhibit KCNJ2 inwardly rectifying potassium channels, depolarizing the cell. The H+ ions may also open unidentified sodium channels to further depolarize the cell. Depolarization causes exocytosis of the neurotransmitters serotonin (5-HT) and gamma-aminobutyric acid (GABA).
Low concentrations of salt appear to be sensed in specific salt-sensing cells that may be a subset of type II cells. Low concentrations of salt are believed to enter the cell through an epithelial sodium channel (ENaC, SCNN) and the ability to taste low concentrations of salt is dependent on the SCNN1A pore-containing subunit of the SCNN complex in mice. Human taste cells express both SCNN1A and SCNN1D pore-containing subunits. The composition of other subunits of the complex is less certain. The transport of sodium ions (Na+) into the cells depolarizes the plasma membrane and eventually leads to opening of CALHM1:CALHM3 channels which transport ATP from the cytosol to the extracellular region.
Sensory perception includes the reactions and physical events that are required to receive a stimulus, convert the stimulus to a molecular signal, and sense the signal. This module includes pathways describing the sensory perception of light (visual transduction, reviewed in Grossniklaus et al. 2015, Molday and Moritz 2015, Lankford et al. 2020), volatile chemicals (olfaction, reviewed in Glezer and Malnic 2019, Lankford et al. 2020), tastants (chemicals that activate taste receptors, reviewed in Roper and Chaudhari et al. 2017), and sound (reviewed in Fettiplace 2017).