GPCRs Flashcards
What does GPCR stand for?
G-protein coupled receptors
What are the major targets of drugs?
Enzymes
Transporters
Ion channels
Receptors
What is a receptor?
target molecules through which soluble physiological mediators can produce their desired biological effects
What are the types of receptors?
GPCR
Ligand gaten ion channels
Enzyme coupled receptors
Nuclear receptors
What is a G-protein coupled receptor?
has seven transmembrane domains
Couples to G-proteins to initiate signal transduction
Activated by diverse signals: photons hormones peptides peptidases
What % of the current drug targets GPCRs
50
What does Ranitidine act on?
histamine H2 receptors for stomach ulcers
What does Loratidine act on
Histamine H1 receptors for allergies
What does Atenolol act on
β-adrenoceptors
for hypertension
WHat does Sumatriptan act on?
5-HT receptors
for migraines
What does Pilocarpine act on?
muscarinic ACh receptors
for glaucoma
What is GPCR Class A?
Class A: Rhodopsin-like
Short N terminus
Have extracellular and intracellular loops
Transmembrane domains
C terminal tail
β-adrenoceptors, histamine receptors, dopamine receptors
What is GPCR Class B?
Class B: Secretin-like
Large globular N terminus (agonist binding)
Have extracellular and intracellular loops
Transmembrane domains
C terminal tail
secretin receptors, calcitonin receptors, glucagon receptors
What is GPCR Class C?
Class C: Metabotropic glutamate/pheromone
Very large N terminus
Have extracellular and intracellular loops
Transmembrane domains
C terminal tail
metabotropic glutamate receptors, GABAB receptors
What is GPCR Class D?
Class D: Fungal pheromone
What is GPCR Class E?
Class E: cAMP receptors
What is GPCR Class F?
Class F: Frizzled/Smoothened
What class do the majority of GPCR drugs target?
A
What do GPCRs couple to?
heterotrimeric G proteins
How are GPCRs activated?
agonist binds to receptor so the receptor undergoes a confirmational change
increasing affinity of receptor for heterotrimeric G proteins which when bound exchange GDP for GTP causing the subunits to dissociate and signal to other effectors
What happens if a GPCR is activated?
Causes signalling events
What does IP3 bind to?
receptors on endoplasmic reticulum
What does the drug salbutamol do?
B2-adrenoreceptors
activating GS subunits
increasing cAMP
increasing effect of PKA
What does the drug Sumatriptan do?
5-HT 1B/D receptors
activate Gi/o subunits
decrease cAMP
decreasing effect of PKA
What does the drug Pilocarpine do?
M3 muscarinic and ACh receptors
Gq subunits
Increase PKC and Calcium ions
What is the desensitisation of GCPRs?
prevents activation of them
usually occurs within seconds
Achieved by phosphorylation and/or internalization
Critical to prevent uncontrolled signaling
A reversible ‘switching off’ of the receptors
What are the two types of GPCR desensitisation?
Homologous
Heterologous
What is homologous GPCR desensitisation?
Homologous Desensitization: effects are restricted to agonists acting through a specific receptor
What is heterologous GPCR desensitisation?
Heterologous Desensitization: effects can affect receptors that share a component of the same signaling cascade
What is the difference in down regulation vs desensitisation?
desensitisation: reduction in the numbers of a functional receptor
Down regulation: is permanent and new receptors must be synthesized i.e., protein synthesis
What do beta arrestins do?
to regulate G protein-coupled receptor (GPCR) signaling and trafficking.
Describe the molecular events that occur after agonist binding that lead to internalization of a GPCR (4 marks)
- Conformational change
- Phosphorylation
- Binding of β-arrestins
- Recruitment of clathrin
G proteins couple to cell-surface receptors to modify the cellular levels of second messenger molecules. Which one of the following molecules is a second messengers?
a) Extracellular-regulated protein kinase (ERK)
b) Protein kinase A
c) 3’-5’-cyclic adenosine monophosphate (cyclic AMP)
d) Gαq
e) Protein kinase C
cAMP
What promotes internalisation of GPCRs?
beta-arrestins
What promotes desensitisation of GPCRs?
phosphorylation
What happens during internalisation of GPCRs?
GPCRs internalize to endosomes and then either recycle (recycling) or are degraded (downregulation)
What is tolerance? Why?
Tolerance is the progressive reduction in the effectiveness of a drug; usually occurring over days or hours
The cellular and molecular mechanisms are not necessarily specified and are often difficult to resolve
Tolerance is a problem in health care industry e.g., morphine
Distinct from the mechanisms involved in desensitization and downregulation
What is an enkaphalin?
- Endogenous ligand
- 5 amino acid peptide (pentapeptide) regulatin regulating nociception
- Activate µ-opioid receptors
What does morphine do?
What is it?
Morphine is an analgesic; activates μ-opioid receptors
Biased agonist of those receptors
What is a biased agonist?
A ‘biased’ agonist differentially activates downstream signaling pathways compared to a ‘standard’ agonist
How does biased agonism of morphine affect signalling and traficking?
Enkephalins and morphine differentially activate downstream signaling and trafficking pathways
Endogenous agonists such as the enkephalins have a high efficacy for endocytosis
Agonists such as morphine have a low efficacy for endocytosis
Enkaphalins activate beta arrestine 1 and 2 what does morphine activate?
Beta arrestine 2
What happens during the development of tolerance at μ-Opioid Receptors? Using enkaphalins?
Normal
GRK desensitization,
β-arrestin
recruitment,
internalization,
resensitization
and recycling
What happens during the development of tolerance at μ-Opioid Receptors? Using morphines?
Enhanced
PKC phosphorylation.
Impaired internalization,
resensitization
and recycling
processes
The development of tolerance to morphine is a major issue for the treatment of pain. Select the best reason why morphine treatment leads to tolerance.
a) morphine promotes degradation of opioid receptors
b) morphine antagonizes opioid receptors
c) morphine promotes a long-lasting desensitization of opioid receptors
d) morphine causes apoptosis
e) morphine activates prevents internalization of opioid receptors
C
Define tolerance at a G protein-coupled receptor (2 marks)
How does it differ from downregulation? (2 marks).
Tolerance is the progressive reduction in the effectiveness of a drug (1 mark), usually occurring over days or hours (1 mark)
Down-regulation is the reduction in the number of functional receptors (1 mark), whereas tolerance does not involve a change in receptor number (1 mark)
