Week 7 - Receptor-Effector signalling

Question 1

How can receptors alter cellular activity?

Answer 1

• Some can alter it directly

- Many require ‘transduction’ of the initial ligand binding event via other intracellular signalling components

Question 2

What are the 3 superfamilies of cell-surface receptors?

Answer 2

• Ligand-gated ion channels
• Receptors with intrinsic enzymatic activity
• G protein-coupled receptors

Question 3

What happens when a ligand binds to a ligand-gated channel?

Answer 3

This activates the receptor

• This directly, or indirectly, brings about a change in cellular activity
• The ‘gates’ open to allow ions to move into or out of the cell

Question 4

What happens when a ligand binds to a receptor with intrinsic enzymatic activity?

Answer 4

It activates an enzyme activity that phosphorylates the receptor itself and other substrates

Question 5

What are agonists?

Answer 5

Molecules that bind to the receptor and activate it

Question 6

What are antagonists?

Answer 6

Molecules that bind to the receptor but DO NOT activate it

Question 7

What are some uses of agonists?

Answer 7

• Anti-asthma: β-2 adrenoceptor agonists, such as salbutamol

- Analgesia/anaesthethia: μ-opioid receptor agonist, e.g. morphine, fentanyl

Question 8

What are some uses of antagonists?

Answer 8

• Cardiovascular: β-adrenoceptor antagonists, e.g. propranolol, atenolol
• Neuroleptics: D2 dopamine receptor antagonists, e/g/ naloperidol, sulpiride

Question 9

What is the effect of a mutation to GPCRs?

Answer 9

Results in loss-of-function or gain-of-function

Question 10

How is familial male precocious puberty caused?

Answer 10

• A GPCR mutation

- Caused by a gain of function mutation to the luteinising hormone receptor

Question 11

What is the common structure of GPCRs?

Answer 11

• Single polypeptide chain (300-1200 amino acids)
• 7 transmembrane spanning regions
• Extracellular N-terminal
• Intracellular C-terminal
• Heterotrimeric (made up of 3 distinct subunits termed α, β and γ - the β and γ subunits bind tightly together to each other and function as a single unit)

Question 12

What can different GPCRs respond to?

Answer 12

• Ions
• Neurotransmitters
• Peptide and non-peptide hormones

Question 13

Which part of the GPCRs can be responsible for ligand binding?

Answer 13

There are 2 regions that can be responsible

• The site may be formed by 2-3 transmembrane domains
• Or the N-terminal region may form the ligand-binding site

Question 14

What happens once a GPCR becomes activated?

Answer 14

It must interact with another protein called a guanine-nucleotide binding protein (G-protein)

• The G-protein α-subunit has a guanine-nucleotide binding site, which binds GDP
• The GPCR-G-protein interaction causes GTP to exchange for GDP on the G-protein α-subunit
• The α-βγ complex immediately dissociates into α-GTP and free βγ subunits
• Each of these subunits can then interact with effector proteins

Question 15

What is the G-protein like in the basal state?

Answer 15

• It is present at the inner face of the plasma membrane
• It is predominantly in its heterotrimeric form
• GDP is bound to the α-subunit

Question 16

How is G-protein signalling terminated?

Answer 16

• The α-GTP and/or βγ interaction with effectors lasts until the α subunit GTPase activity hydrolyses GTP back to GDP
• α-GDP and βγ subunits then reform an inactive G-protein

Question 17

What governs receptor-G-protein selection?

Answer 17

• Activated GPCRs preferentially interact with specific types of G-protein (the Gα subunit is a primary determinant)
• In turn, the Gα subunits and Gβγ subunits interact with specific effector proteins

Question 18

If acetylcholine binds to an M2-muscarinic receptor, what is the G-protein, effect and physiological response?

Answer 18

• Gi
• Inhibits adenylyl cyclase, stimulates K+ channel
• Slows cardiac pacemaker

Question 19

If adrenaline binds to a β-adrenoceptor, what is the G-protein, effect and physiological response?

Answer 19

• Gs
• Stimulates adenyl cyclase
• Causes glycogenolysis and lipolysis

Question 20

If acetylcholine binds to an M3-muscarinic receptor, what is the G-protein, effect and physiological response?

Answer 20

• Gq
• Stimulates phospholipase C
• Causes smooth muscle contraction

Question 21

If light stimulates the rhodopsin receptor, what is the G-protein, effect and physiological response?

Answer 21

• Gt
• Stimulates cyclic GMP phosphodiesterase
• Causes visual excitation

Question 22

What are the different effector types?

Answer 22

• Enzymes
• Ion channels
• Adenylyl cyclase
• Phospholipase C

Question 23

Give some examples of enzymes as effectors

Answer 23

• ATP can be converted to cyclic AMP by adenylyl cyclase
• PIP2 can be converted to IP3 and DAG by phospholipase C
• PIP2 can be converted to PIP3 by phosphoinositide 3-kinase
• Cyclic GMP can be converted to 5’-GMP by cGMP phosphodiesterase

Question 24

Give some examples of ion channels as effectors

Answer 24

• Voltage-operated Ca2+ channels

- G protein-regulated inwardly-rectifying k+ channels

Question 25

Give some examples of GPCRs that use agonist-stimulated regulation of adenylylcyclase

Answer 25

Gs-coupled receptors
- β-adrenoceptors
- D1 dopamine receptors
- H2 histamine receptors
Gi coupled receptors
- α2-adrenoceptors
- D2 dopamine receptors
- μ-opioid receptors

Question 26

How does phospholipase C act as an effector?

Answer 26

It catalyses the cleavage of the membrane phospholipase PIP2 into 2 second messengers

• IP3
• DAG

Question 27

What are some examples of GPCRs that use phospholipase C as an effector?

Answer 27

Gq coupled receptors

• α1-adrenoceptors
• M1 muscarinic receptors
• H1 histamine receptors

Question 28

Give an example of signal amplification in a signalling pathways

Answer 28

Adrenaline

• A few molecules of adrenaline binding to cell surface β-adrenoceptors may cause a relatively massive cellular response
• The β-adrenoceptor –> Gs protein –> adenylyl cyclase part of the cascade causes relatively little amplification
• Activation of adenylyl cyclase generates many molecules of cyclic AMP
• These then activate the enzyme PKA

Question 29

How do signal transduction pathways cause inotrophy in the heart?

Answer 29

Both blood-borne adrenaline and sympathetically released noradrenaline can interact with ventricular β1-adrenoceptors to increase the force of contraction

• Uses the adenylyl cyclase pathway
• The cyclic AMP-dependent protein kinase activates voltage-gated Ca2+ channels
• So more Ca2+ enters the cell in each depolarisation
• Hence there is increase contractility

Question 30

How do signal transduction pathways cause smooth muscle contraction?

Answer 30

• Sympathetically released noradrenaline can interact with vascular smooth muscle α1-adrenoceptors to cause vasoconstriction
• Parasympathetically released acetylcholine can interact with bronchiolar smooth muscle M3-muscarinic receptors to cause bronchoconstriction
• A variety of agents acting at GPCRs can contract GI and genitourinary smooth muscle
• All utilise the Gq-phospholipase C-IP3/Ca2+, DAG/protein kinase C pathways

Question 31

How do signal transduction pathways modulate neurotransmitter release?

Answer 31

In both the CNS and PNS, neurotransmitter release is often modulated by presynaptic GPCRs
- Gβγ subunits inhibit specific types of voltage-operated Ca2+ channels, reducing Ca2+ influx and hence neurotransmitter release