Taste Flashcards
What are our five tastes?
- Bitter: toxins (from plants)
- Sweet: sucrose (energy source)
- Umami: monosodium glutamate (components of protein)
- Sour: protons, citrus, acids
- Salty: NaCl
Which tastes are aversive? attractive?
aversive:
bitter, sour
attractive:
sweet, umami, salty
What is the structure of a taste receptor?
- taste receptor cells do not have axons,
- innervated by fibers originating in taste ganglia that travel in cranial nerves
2 characteristics of taste buds?
- 50-100 taste receptor cells per taste bud
- regenerate continuously throughout adult life
How are different tastes detected? (2)
- specific receptors detect tastants
2. specific subsets of cells respond to each class of tastants
What was the experiment used to identify the sweet receptor?
- cloned from mouse based on identification of mouse that cannot taste sweet
- respond to sweet using calcium imaging
In the mouse used to identify sweet receptor, what was the role of calcium?
Ca2+ elevation in response to sweet
What are the 2 sweet response receptors?
T1R2 and T1R3 coupled to PLCB2 and TRPM5
What is the umami receptor composed of?
T1R1 + T1R3
Are receptors Ca2+ permeable?
no
What is the cause of the Ca2+ elevation?
- due to coupling of taste receptor to signaling pathway that activate IP3 receptors
- release Ca2+ from intracellular stores
What are the bitter receptors?
-T2Rs
What are 3 qualities of bitter receptors?
- family of 30 GPCRs
- all expressed in same cells
- cannot distinguish different bitter tastes
What is the transduction pathway for bitter taste? (8)
- Ligand binds to receptor, activates G protein
- activates PLC
- breaks down PIP2 to IP3 + DAG
- IP3 opens IP3 receptor/Ca2+ on ER
- IC Ca2+ increases
- opens TRPM5 ion channel
- lets in Na2+
- depolarizes taste cells
Which receptor types are unique to taste cells?
TRPM5 and PLCB2
Can TPRM5 and PLCB2 Knockouts taste bitter or sweet taste?
no
Taste Coding (2 models)
- Cells dedicated to one of 5 tastes, signaling to brainn is hardwired
- Cells express mix of receptors, brain sorts it all out
What is the PLCB2 rescue experiment?
- Eliminate all signaling by removing PLCB2 (animal cannot taste, biter, sweet, umami)
- Add PLCB2 back under promotor of bitter receptor (transgenic mouse)
- Ask: Does this restore sensitivity to sweet?
What does yes/no mean for PLCB2 rescue experiment?
No: cells that make bitter are different from cells that make sweet (labelled line)
Yes: some cells make bitter and sweet, FALSE
What are the 2 elements introduced into a transgenic mouse?
- promoter
2. transgene
What determines whether something tastes good or bad?
-Cell type, not receptor
What experiment showed what determines whether something tastes good/bad?
- inserted unnatural receptor (RASSL) into T2R expressing cells
- animals avoid agonist
What happens when human specific bitter receptor is put into T2R cell?
-animal avoid biter agonist
What happens if human bitter receptor in T1R cell?
-animal attracted to bitter
How are genetic changes made?
promoters
What is the T1R1 promoter?
-T1R1 promoter directs gene to be expressed in umami cell
What is the T1R2 promoter?
-directs gene to be expressed in sweet cell
What is the T2R promoter?
-directs genes to be expressed in bitter cell
What is so interesting about PTC?
-There is polymorphism of whether or not people can taste it.
Where is the variation for PTC?
-Tas2R38 gene (PAV is taster and AV1 is non taster)
What is evidence for balancing selection?
- both tasters and nontasters are found in equal proportions in the populations
- both alleles must have some selective advantage
Are tasters or nontasters dominant?
tasters (Mendelian Inheritance)
Why can’t cats taste sweet?
-T1R2 is a pseudogene in cats meaning it contains inactivating mutations
EX: cats are obligate carnivores
What are sour and salty receptors?
- Both are ion channels
- do not use PLCB2 or TRPM5.
