Taste

Question 1

What are our five tastes?

Answer 1

1. Bitter: toxins (from plants)
2. Sweet: sucrose (energy source)
3. Umami: monosodium glutamate (components of protein)
4. Sour: protons, citrus, acids
5. Salty: NaCl

Question 2

Which tastes are aversive? attractive?

Answer 2

aversive:
bitter, sour
attractive:
sweet, umami, salty

Question 3

What is the structure of a taste receptor?

Answer 3

• taste receptor cells do not have axons,

- innervated by fibers originating in taste ganglia that travel in cranial nerves

Question 4

2 characteristics of taste buds?

Answer 4

• 50-100 taste receptor cells per taste bud

- regenerate continuously throughout adult life

Question 5

How are different tastes detected? (2)

Answer 5

1. specific receptors detect tastants

2. specific subsets of cells respond to each class of tastants

Question 6

What was the experiment used to identify the sweet receptor?

Answer 6

• cloned from mouse based on identification of mouse that cannot taste sweet
• respond to sweet using calcium imaging

Question 7

In the mouse used to identify sweet receptor, what was the role of calcium?

Answer 7

Ca2+ elevation in response to sweet

Question 8

What are the 2 sweet response receptors?

Answer 8

T1R2 and T1R3 coupled to PLCB2 and TRPM5

Question 9

What is the umami receptor composed of?

Answer 9

T1R1 + T1R3

Question 10

Are receptors Ca2+ permeable?

Answer 10

no

Question 11

What is the cause of the Ca2+ elevation?

Answer 11

• due to coupling of taste receptor to signaling pathway that activate IP3 receptors
• release Ca2+ from intracellular stores

Question 12

What are the bitter receptors?

Answer 12

-T2Rs

Question 13

What are 3 qualities of bitter receptors?

Answer 13

1. family of 30 GPCRs
2. all expressed in same cells
3. cannot distinguish different bitter tastes

Question 14

What is the transduction pathway for bitter taste? (8)

Answer 14

1. Ligand binds to receptor, activates G protein
2. activates PLC
3. breaks down PIP2 to IP3 + DAG
4. IP3 opens IP3 receptor/Ca2+ on ER
5. IC Ca2+ increases
6. opens TRPM5 ion channel
7. lets in Na2+
8. depolarizes taste cells

Question 15

Which receptor types are unique to taste cells?

Answer 15

TRPM5 and PLCB2

Question 16

Can TPRM5 and PLCB2 Knockouts taste bitter or sweet taste?

Answer 16

no

Question 17

Taste Coding (2 models)

Answer 17

1. Cells dedicated to one of 5 tastes, signaling to brainn is hardwired
2. Cells express mix of receptors, brain sorts it all out

Question 18

What is the PLCB2 rescue experiment?

Answer 18

1. Eliminate all signaling by removing PLCB2 (animal cannot taste, biter, sweet, umami)
2. Add PLCB2 back under promotor of bitter receptor (transgenic mouse)
3. Ask: Does this restore sensitivity to sweet?

Question 19

What does yes/no mean for PLCB2 rescue experiment?

Answer 19

No: cells that make bitter are different from cells that make sweet (labelled line)
Yes: some cells make bitter and sweet, FALSE

Question 20

What are the 2 elements introduced into a transgenic mouse?

Answer 20

1. promoter

2. transgene

Question 21

What determines whether something tastes good or bad?

Answer 21

-Cell type, not receptor

Question 22

What experiment showed what determines whether something tastes good/bad?

Answer 22

• inserted unnatural receptor (RASSL) into T2R expressing cells
• animals avoid agonist

Question 23

What happens when human specific bitter receptor is put into T2R cell?

Answer 23

-animal avoid biter agonist

Question 24

What happens if human bitter receptor in T1R cell?

Answer 24

-animal attracted to bitter

Question 25

How are genetic changes made?

Answer 25

promoters

Question 26

What is the T1R1 promoter?

Answer 26

-T1R1 promoter directs gene to be expressed in umami cell

Question 27

What is the T1R2 promoter?

Answer 27

-directs gene to be expressed in sweet cell

Question 28

What is the T2R promoter?

Answer 28

-directs genes to be expressed in bitter cell

Question 29

What is so interesting about PTC?

Answer 29

-There is polymorphism of whether or not people can taste it.

Question 30

Where is the variation for PTC?

Answer 30

-Tas2R38 gene (PAV is taster and AV1 is non taster)

Question 31

What is evidence for balancing selection?

Answer 31

- both tasters and nontasters are found in equal proportions in the populations - both alleles must have some selective advantage

Question 32

Are tasters or nontasters dominant?

Answer 32

tasters (Mendelian Inheritance)

Question 33

Why can’t cats taste sweet?

Answer 33

-T1R2 is a pseudogene in cats meaning it contains inactivating mutations EX: cats are obligate carnivores

Question 34

What are sour and salty receptors?

Answer 34

- Both are ion channels | - do not use PLCB2 or TRPM5.