Week 7 - Receptor-Effector signalling Flashcards
How can receptors alter cellular activity?
- Some can alter it directly
- Many require ‘transduction’ of the initial ligand binding event via other intracellular signalling components
What are the 3 superfamilies of cell-surface receptors?
- Ligand-gated ion channels
- Receptors with intrinsic enzymatic activity
- G protein-coupled receptors
What happens when a ligand binds to a ligand-gated channel?
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
What happens when a ligand binds to a receptor with intrinsic enzymatic activity?
It activates an enzyme activity that phosphorylates the receptor itself and other substrates
What are agonists?
Molecules that bind to the receptor and activate it
What are antagonists?
Molecules that bind to the receptor but DO NOT activate it
What are some uses of agonists?
- Anti-asthma: β-2 adrenoceptor agonists, such as salbutamol
- Analgesia/anaesthethia: μ-opioid receptor agonist, e.g. morphine, fentanyl
What are some uses of antagonists?
- Cardiovascular: β-adrenoceptor antagonists, e.g. propranolol, atenolol
- Neuroleptics: D2 dopamine receptor antagonists, e/g/ naloperidol, sulpiride
What is the effect of a mutation to GPCRs?
Results in loss-of-function or gain-of-function
How is familial male precocious puberty caused?
- A GPCR mutation
- Caused by a gain of function mutation to the luteinising hormone receptor
What is the common structure of GPCRs?
- 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)
What can different GPCRs respond to?
- Ions
- Neurotransmitters
- Peptide and non-peptide hormones
Which part of the GPCRs can be responsible for ligand binding?
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
What happens once a GPCR becomes activated?
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
What is the G-protein like in the basal state?
- 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
How is G-protein signalling terminated?
- 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
What governs receptor-G-protein selection?
- 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
If acetylcholine binds to an M2-muscarinic receptor, what is the G-protein, effect and physiological response?
- Gi
- Inhibits adenylyl cyclase, stimulates K+ channel
- Slows cardiac pacemaker
If adrenaline binds to a β-adrenoceptor, what is the G-protein, effect and physiological response?
- Gs
- Stimulates adenyl cyclase
- Causes glycogenolysis and lipolysis
If acetylcholine binds to an M3-muscarinic receptor, what is the G-protein, effect and physiological response?
- Gq
- Stimulates phospholipase C
- Causes smooth muscle contraction
If light stimulates the rhodopsin receptor, what is the G-protein, effect and physiological response?
- Gt
- Stimulates cyclic GMP phosphodiesterase
- Causes visual excitation
What are the different effector types?
- Enzymes
- Ion channels
- Adenylyl cyclase
- Phospholipase C
Give some examples of enzymes as effectors
- 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
Give some examples of ion channels as effectors
- Voltage-operated Ca2+ channels
- G protein-regulated inwardly-rectifying k+ channels
Give some examples of GPCRs that use agonist-stimulated regulation of adenylylcyclase
Gs-coupled receptors - β-adrenoceptors - D1 dopamine receptors - H2 histamine receptors Gi coupled receptors - α2-adrenoceptors - D2 dopamine receptors - μ-opioid receptors
How does phospholipase C act as an effector?
It catalyses the cleavage of the membrane phospholipase PIP2 into 2 second messengers
- IP3
- DAG
What are some examples of GPCRs that use phospholipase C as an effector?
Gq coupled receptors
- α1-adrenoceptors
- M1 muscarinic receptors
- H1 histamine receptors
Give an example of signal amplification in a signalling pathways
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
How do signal transduction pathways cause inotrophy in the heart?
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
How do signal transduction pathways cause smooth muscle contraction?
- 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
How do signal transduction pathways modulate neurotransmitter release?
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
