W/S 1 - Receptor resensitisation

Question 1

Receptor sequestration

Answer 1

receptors on the surface of a cell are internalized

Question 2

S. S. Yu, 1993 - alternative model of sequestration

Answer 2

altered conformation in plasma membrane with different ligand-binding properties

Question 3

S. S. Yu, 1993 - sequestration being blocked

Answer 3

via mutagenesis of the receptor

substitute the 4 serine and threonine residues in the cytoplasmic C terminus tail of receptor with alanine and glycine residues

Question 4

Koch et al, 1998 - rat mu opioid receptor isoforms

Answer 4

MOR1 + MOR1B

Differ in length and amino acid composition at carboxyl terminus

both bind to mu receptor agonist (DAMGO)

Question 5

Koch et al, 1998 - MOR1B properties

Answer 5

1. Desensitised at slower rate
2. Resensitised at faster rate during agonist withdrawal
3. DAMGO-induced internalisation proceeded faster followed by rapid recycling of receptor to cell surface

Question 6

Koch et al, 1998 - reason for MOR1B properties

Answer 6

sequence at cytoplasmic tail of MOR1B facilitates clathrin-coated vesicle-mediated endocytosis - promotes accelerated receptor activation (carboxyl-terminal splicing)

MOR1B lacks threonine 394 - more resistant to agonist-induced desensitisation

Question 7

Koch et al, 1998 - monesin

Answer 7

inhibits endosomal acidification (blocks resensitisation)

Question 8

Koch et al, 1998 - sucrose

Answer 8

disrupts formation of clathrin-coated vesicle pits (blocks internalisation)

Question 9

Constitutive internalisation

Answer 9

receptors continuously internalised from cell surface even in absence of ligand

Question 10

Koch et al, 1998 - naloxone (mu receptor antagonist)

Answer 10

led to increase of Bmax of MOR1B (not MOR1), supporting idea of constitutive internalisation of MOR1B receptor

Question 11

process of receptor resensitisation

Answer 11

[desensitisation]
- receptors exposed to persistent stimulus (= reduced responsiveness)
- mediated by PKA (cAMP produced by adenylate cyclase activates PKA)

[sequestration]
-uncoupled receptors internalised via endocytosis

[dephosphorylation]
-reverse modifications that led to desensitisation (low pH)

[resensitisation/recycling]
- re-inserted into plasma membrane + interact with ligands again

OR

degradation via lysosomes

Question 12

agonist binding to GPCR

Answer 12

leads to receptor phosphorylation by BARK/PKA

(beta-arrestin binding to ligand)

Question 13

S. S. Yu, 1993 - proposed desensitisation mechanisms

Answer 13

1. agonist occupancy = phosphorylation of B2-adrenergic receptor (BAR) via BAR kinase + PKA
   => disrupts interaction with Gs = reduced adenylyl cyclase activation
2. receptor sequestration (reduced no. of receptors)

Question 14

S. S. Yu, 1993 - evidence against sequestration theory

Answer 14

sequestration blocked by ConA or phenylarsine had no effect on desensitisation

Question 15

Koch et al, 1998 - results from study

Answer 15

no. of binding sites = more in MOR1 vs MOR1B

80% MOR1 left after 4hrs

40% MOR1B left after 2hrs then plateau

ability of MOR1B to inhibit cAMP accumulation = strongly decreased after 1hr DAMGO exposure

Question 16

proposed concept of intrinsic efficacy

Answer 16

[Furchgott]

amount of efficacy (e) transmitted by interaction of drug with each receptor

Question 17

Parra, S + Bond, R (2007) - old classification

Answer 17

full agonists, partial agonists and antagonists

= too simplistic

Question 18

Parra, S + Bond, R (2007) - collateral efficacy

Answer 18

unintended but potentially beneficial effects of drug arising from its action on targets other than primary target

Question 19

Parra, S + Bond, R (2007) - collateral efficacy EXAMPLE

Answer 19

certain cholecystokinin antagonists can promote receptor internalisation without activating any agonist response

Question 20

constitutive receptor binding

Answer 20

measurable activity under basal conditions

Question 21

constitutive activity - inverse agonist vs neutral antagonist

Answer 21

decreases vs blocks

Question 22

action - inverse agonist vs neutral antagonist

Answer 22

enrichment of inactive state vs don’t have capacity to activate/deactivate active state

Question 23

Parra, S + Bond, R (2007) - therapeutic use of inverse agonist EXAMPLE

Answer 23

[heart failure]

carvedilol + metoprolol = beta-blockers (B1-AR inverse agonist)

limit excessive adrenergic stimulation

shown to reduce mortality and number of hospitalized patients

Question 24

therapeutic route for agonists

Answer 24

beneficial acute response (increased active receptors)

phosphorylation of GRKs, endocytosis, downregulation of receptors

reduced chronic response (= detrimental)

Question 25

therapeutic route for inverse agonists

Answer 25

detrimental acute response (increased inactive receptors)

prevents phosphorylation + upregulation of receptors

chronic paradoxical benefits