Taste signalling Flashcards
How many transmembrane helices in GPCRs?
7
Define taste
taste is used to describe sensations arising predominantly from the oral cavity
What type of signal is the sense of taste?
a chemosensory signal mediated by the tongue
Taste buds open into the oral cavity through a __ __
taste pore
Taste buds are most prevalent on epithelium of the tongue called ____
papillae - although also present on palate, pharynx, and larynx
Where is the highest conc of taste buds?
side and back of tongue
What papillae are present on the anterior part of the tongue?
Fungiform papillae, which contain 0-15 taste buds
What papillae are present on the posterior portion of the tongue? How many taste buds do they contain?
Foliate and circumvallate papillae contain dozens of tastebuds
2 types of cells in taste buds
- basal cells - small round cells at the base of taste buds
- elongated cells - stretch from base to apical end, possess microvilli and have 3 types [I (glial like), II and III (presynaptic cells)]
Taste receptor cells (TRCs) are what kinds of taste bud cells?
Type II cells (elongated)
If taste sensation is lost, it returns quickly. Why?
Due to high turnover of TRCs. Avg lifespan = 10-11 days
5 conventional tastes
sweet, bitter, sour, salt, and umami
What does each taste indicate?
- Sweet - indicates energy-rich nutrients (carbs)
- Bitter - allows sensing of natural toxins
- Sour - the taste of acids
- salt - For electrolyte balance
- Umami - the taste of amino acids (eg. meat broth)
What are you actually sensing when eating spicy foods?
Sensing pain (nociception). CAPSAICIN activates nociceptors
When you see T2, you know that it’s for __ receptors
bitter
Receptors for sweet sensation
T1R2 + T1R3
Receptors for umami
T1R1 + T1R3
Receptor for bitter
T2R
Two new taste-coding mdoels
- labelled-line model: each TRC responds to single taste modality
- Across-fiber model: Each TRC responds to multiple taste modalities
What molecule is increased when sugar is eaten? What about when something bitter is eaten?
sweet = cAMP bitter = calcium
T/F: humans can taste longer chain oligo- and polysaccharides
false. Humans can only taste mono and disaccharides
what occurred when sweet taste receptor was blocked by lactisole (a known sweet inhibitor)? What does this suggest
Subjects could not detect sweet substances (glucose, matose, sucralose), but they could still detect the glucose oligomers.
This suggests that glucose oligomer detection is independent of T1R2/T1R3 sweet taste receptor. Oigomers were described as “starchy”
Which taste receptor gene is expressed in ALL types of taste buds?
T1R3
Which taste receptor gene is predominantly expressed in fungiform and palate taste buds, less so in foliate taste buds, and rarely in circumvallate taste buds?
T1R1
T1R2 gene is predominantly expressed in _____ and ___- taste buds
circumvallate and foliate. Less so in palate taste buds and rarely in fungiform taste buds
which two taste receptor genes are rarely coexpressed in the same TRC?
T1R1 and T1R2
What has bitter taste evolved as?
a central warning system against the ingestion of potentially toxic substances
Are T1R genes coexpressed with T2R genes in a single TRC?
No
Position of ligand binding in T1R vs T2R receptors
for T2Rs, ligand binds inside the helices.
For T1Rs, it binds to its N-terminus (has a longer N-terminus compared to T2Rs)
Umami is the taste of _____
MSG
The receptors that encode umami taste include both ___ and ___
GPCRs and NMDR receptor ion channels
Salt taste sensation is mediated by:
ion channels
Flavour = ___ + ___
taste + olfaction
What percentage of GPCRs are for olfaction in humans?
50%
efficacy of a compound refers to what?
its ability to activate receptors to a degree
How many T2Rs have been identified in the human genome?
25
T2Rs in the gastric and intestinal mucosa do what?
act as a second line of defence that determine whether ingested substances are beneficial or harmful. They either initiate digestion and absorption or induce vomiting and aversive behaviour
What happens when bitter compounds enter the lungs?
bitter compounds increase the intracellular calcium ion concentration and stimulate ciliary beat frequency. This is a defense mechanism to eliminate the offending compound.
How are bitter compounds used in asthma?
bitter tastants cause RELAXATION of isolated airway smooth muscle and dilation of arways. Bitter tastants decreased airway obstruction
The human bitter receptor, T2R38, is known as the ___ receptor
PTC (phenylthiocarbamide)
Humans are categorized into tasters and non-tasters based on their ability to taste ___
PTC. Ability to taste PTC is a dominant genetic trait
The two forms of T2R38, designated as taster and non-taster, differ from each other how?
at three amino-acid positions
Genotypes of tasters, non-tasters, and supertasters
Supertasters: PAV/PAV
Taster: PAV/AVI
Nontaster: AVI/AVI
what happens in homozygous PAV/PAV individuals
they have the ability to sense PTS and PROP as intensely bitter
What 3 amino acids are contained by a functional T2R38?
PAV: proline, alanine and valine
Nasal chemosensory cells use bitter taste signaling to detect:
irritants and bacterial signals
Inhalation of a bitter antibiotic can have additional effects?
yes. Could activate bitter receptors, increasing intracellular calcium and possibly an inflammatory response
What kind of papillae are present in more abundance in supertasters?
fungiform
Single nuceotide changes/mutations are known as
single nucleotide polymorphism (SNP)
What are sSNPs and nsSNPs?
sSNPs (synonymous) are due to mutation in DNA sequence but not in amino acid sequence.
nsSNPs (non-synonymous) are due to mutation in DNA sequene that causes change in amino acid sequence.
nsSNPs in T2R38 and T1R2 is associated with what?
higher caries risk in children
Relationship between PAV/PAV genotype and rheumatoid arthritis
Sueprtasters (PAV/PAV) have an enhanced immune response and fight disease better. This becomes a problem with age, as the immune system may become “rogue” and cause autoimmune disorders such as RA
Do supertasters have a greater or lesser risk of childhood caries?
lesser
Why can’t you feed dogs xylitol?
Because dogs produce insulin when they eat xylitol (humans do not) and can become hypoglycemic. Could be deadly
Antagonist or inverse agonists of T2Rs are referred to as ____
bitter blockers or bitter taste blockers
Fade vs tachyphylaxis vs desensitization
Fade: decline in response in the CONTINUED presence of agonist
Tachyphyllaxis: decline in response to REPEATED application of agonist
Desensitization: fade or tachyphylxis in a direct consequence of receptor activation
these are overlapping terms
What causes taste signal termination?
arrestins bind to the desensitized receptors and uncouple the bound G-protein
2 possible mechanisms of T2R desensitization:
- Receptor desensitization
- activation of arrestin
Aging as a factor affecting taste perception
with age, number of taste buds in the oral cavity decrease
How does smoking affect taste?
causes taste loss due to desensitization of bitter receptors
What is geusia?
sense of taste
3 main types of geusia
- Ageusia: complete lack of taste
- Hypogeusia: deacreasi in taste sensitivity
- Dysgeusia: distortion/changes in taste sensitivity
what kind of supplements have been shown to help patients with dysgeusia?
zinc supplementation
What symptoms related to taste do patients with familial dysautonomia display?
inability to taste.
Absence of taste buds and a paucity of sensory nerves in the tongue
What is Bell’s palsy and how does it affect taste?
Bell’s palsy is a form of temporary facial paralysis resulting from damage to the facial nerve due to viral infection by viral meningitis or herpes simplex.
Symptoms may include dry mouth and impairment of taste mostly on one side of the tongue
What is Sjogren’s syndrome?
an autoimmune disorder in which immune cells attack and destroy glands that produce tears and saliva.
What is Wallenberg’s syndrome?
neurological condition caused by a stroke. Symptoms include dicfficulty swallowing, nausea and vomiting. Some individuals LOSE THEIR SENSE OF TASTE ON ONE SIDE OF THE TONGUE, while preserving taste on the other side.
Which nerve can be damaged during a third molar extraction and what affect can this have?
chorda tympani. Damage can lead to temporary loss of taste perception (up to 6 months)
