GPCRs 3 Flashcards
1
Q
What is endocytosis
A
- Arrestin brings AP-2 which brings clathrin with it to cell membrane
- Clathrin oligomerises and become big chain
- Clathrin pinches off bit of membrane- invagination
- Dynamin is attracted to and pinches off neck of invagination producing a clathrin coated vesicle
2
Q
What happens to the clathrin coated vesicle once it has formed
A
- Clathrin coated vesicle then fuses to early endosome – assumption that agonist unbinds and phosphate groups drop off
- Acidic environment in vesicle
- Agonist binds to receptor in Ph sensitive manner
- Then two separate pathways depending on type of receptor
a. Late endosome - most
b. Lysosome
3
Q
What happens in the late endosome
A
- Way of resensitising and recycling receptor after desensitisation
4
Q
What happens in the lysosome pathway
A
- Receptor gets metabolised/degraded – physically removed from cell
- Have to wait for new receptors to be synthesised to replace desensitised receptors
5
Q
If receptor can desensitise by two mechanisms which causes internalisation
A
- If receptor can desensitise by two mechanisms, only really arrestin desensitisation causes internalisation
- DAMGO on mu-opiod receptor would not have the same effect
6
Q
What is the functional role of internalization?
A
- Some internalized GPCRs are rapidly recycled- late endosome: eg. b2, a, m-opioid, D1
- Others are degraded (down-regulated)- lysosome:
eg. AT1, neurotensin, P2Y, NK1, d-opioid - Downregulation will reduce GPCR signalling but does internalization always reduce GPCR signalling?
- Constitutive internalization - Inverse agonists
7
Q
Give example of when inverse agonists could be used to reduce desensitisation
A
- B2 adrenoceptor
- Targeted
- Causes bronchodilation
- Tolerance is problem
8
Q
How can inverse agonists reduce desensitisation problem in asthma medication
A
- In-vivo assay of asthma
- Mice sensitised so methacholine causes airway resistance- induces asthma attack
- Reduced airway resistance when given salbutamol and alprenolol
- After 28 days sal and alpren were ineffective – receptors become desensitised
- Alprenolol decreases asthma – day 1
- Carvedilol (Beta blockers) - Antagonist increases severity to asthma attack – day 1
- Day 28 Alp is no longer effective
- Beta blockers (antagonist…)- decrease severity of asthma after 28 days
- Actually inverse agonists – very very weak
- Stabilise inactive state of receptor
- Chronic treatment with an inverse agonist – huge increase in cell surface receptor density after 28days being treated by inverse agonist
9
Q
What is reason for inverse agonists reducing desensitisation problem
A
- GPCRs show constitutive internalisation – very small proportion spontaneously internalise- small proportion degrade
- But get replaced by new receptor synthesised
- Inverse agonist stabilises inactive state which prevents It from being constitutively internalised and degraded
- Still getting synthesised
- Leads to increase in receptor density
10
Q
What is problem with inverse agonists with asthma treatment
A
- Increases severity of asthma – so problematic
- Could with nu-opiod receptor- build up receptor density before surgery etc
11
Q
Do receptors exist in isolation
A
- Receptors don’t exist in isolation – idea that GPCRs interact with each other
- Hardly ever find mammalian cell which only expresses one type of GPCR
- Activation of one receptor can affect how the other one acts. eg. heterologous desensitization
12
Q
Give example of heterologous desensitisation
A
- Receptor B having impact on the function of receptor A via a cellular mechanism
- e.g. PKC signalling and its subsequent effects on mu-opioid receptors.
13
Q
What is oligomerisation
A
- Oligomerization: when two or more receptors are PHYSICALLY linked
- Ion channels are generally oligomers (eg. GABAA, P2X) and don’t function as monomers
- GPCRs can function as monomers unlike ion channels, but can form dimers with different pharmacology
14
Q
What is a Homodimer
A
- two of the same receptors oligomerised
15
Q
What is a heterodimer
A
- two different GPCRs linked together
