Signal transduction Flashcards
Why is signal transduction important?
Although some receptors can directly alter cellular activity, many require “transduction” of the initial ligand binding event via other intracellular signalling components to generate a response, e.g. contraction, secretion, proliferation, differentiation, etc.
Give 2 examples of intracellular receptors.
Steroid hormone receptors, thyroid hormone receptors
Why are most receptors located on the cellular surface?
The majority of extracellular signalling molecules do not readily cross the plasma membrane
Name 3 extracellular signalling molecules.
Hormones, neurotransmitters, growth factors
What needs to be present in order for cells to respond to extracellular signals?
The appropriate receptor
What are the 3 steps in signal transduction?
Reception
Transduction
Response
What are the 3 superfamilies of cell surface receptor?
• Ligand-gated (receptor-operated) ion channels
(e.g. nicotinic acetylcholine receptors)
• Receptors with intrinsic enzymatic activity
(receptor tyrosine kinases (e.g. insulin receptor)
• G protein-coupled (7TM) receptors
(e.g. muscarinic acetylcholine receptors)
What does ligand binding do to a receptor?
Activates the receptor, which in turn directly or indirectly
brings about a change in cellular activity
How do ligand gated ion channels work?
Ligand binding „gates‟ the channel to allow ions to move into or out of the cell
How do receptors with intrinsic enzymatic activity work?
Ligand binding activates an enzyme activity (e.g. tyrosine
kinase) that phosphorylates the receptor itself + other substrates
Describe the insulin receptor.
Heterotetramer (2 alpha and 2 beta subunits)
Alpha subunit- insulin binding
Beta subunits contain enzymatic activity
Phosphorylate tyrosine residues on each other when insulin binds
Why, clinically, is it important to know about how GPCRs work?
Currently ~40% of all available prescription drugs exert their therapeutic effects directly (as agonists or antagonists) or indirectly at GPCRs
What does an agonist do to a GPCR?
Bind to the receptor and activate it
leading to intracellular signal transduction events
What does an antagonist do to a GPCR?
Bind to the receptor but do not activate it
block the effects of agonists at the receptor
How are GPCR agonists used therapeutically?
Anti-asthma: β2 adrenoceptor agonists SALBUTAMOL, SALMETEROL Analgesia/anaesthesia: μ-opioid receptor agonists MORPHINE, FENTANYL
How are GPCR antagonists used therapeutically?
Cardiovascular (e.g. hypertension): β adrenoceptor antagonists PROPRANOLOL, ATENOLOL Neuroleptics (e.g. anti-schizophrenic): D2 dopamine receptor antagonists HALOPERIDOL, SULPIRIDE
What is clopidogrel?
An anti platelet drug (newish)
Used for coronary artery disease/ MI prevention
Irreversible antagonist at PY12 purinoceptor
Warnings being flagged- may have some problems
What is aripiprazole?
Anti schizophrenic (atypical). Also used in unipolar and bipolar depression D2Rs (partial agonist- weak agonistic action)
What are the therapeutic applications of drugs targeting GPCRs in the CNS?
Depression Schizophrenia Psychosis Parkinson‟s disease Migraine
What are the therapeutic applications of drugs targeting GPCRs in the CVS?
Hypertension
Congestive heart failure
Cardiac arrhythmia
Thrombosis
What are the therapeutic applications of drugs targeting GPCRs in the respiratory system?
Asthma
Chronic obstructive pulmonary disease (COPD)
What are the therapeutic applications of drugs targeting GPCRs in the gastrointestinal system?
Acid reflux
Gastric ulcer
Nausea
What are the therapeutic applications of drugs targeting GPCRs in the genitourinary system?
Overactive bladder
Prostate cancer
Benign prostatic hyperplasia
What are other general therapeutic applications of drugs targeting GPCRs?
Chronic pain Glaucoma Rhinitis Motion sickness Anaphylaxis
What do mutations to G protein-coupled receptors result in? Give 3 examples.
Genetic changes to GPCRs result in loss-of-function or gain-of-function mutations, e.g.
- Retinitis pigmentosa can be caused by a loss-of-function mutation to rhodopsin
- Nephrogenic diabetes insipidus can be caused by a loss-of-function mutation to the V2 vasopressin receptor
- Familial male precocious puberty is caused by a gain-of-function mutation to the luteinizing hormone (LH) receptor
What are the different stimuli that GPCRs can respond to?
-Sensory GPCRs sense light (e.g. rhodopsin), odours and tastes
Different GPCRs can also respond to:
• Ions (H+, Ca2+)- these may also be called proton/ acid sensing GPCRs
• Neurotransmitters (e.g. acetylcholine, glutamate, dopamine, GABA)
• Peptide and non-peptide hormones (e.g. glucagon, adrenaline)
• Large glycoproteins (e.g. thyroid-stimulating hormone (TSH))
How many different GPCRs have been identified in the human genome?
Over 800 ( in lecture he said around 865). This accounts for over 2% identified genes.
What is the basic structure of all GPCRs?
- Single polypeptide chain (300-1200 amino acids)
- 7-transmembrane (7TM)- spanning regions
- Extracellular N-terminal
- Intracellular C-terminal
Where are the 2 regions in GPCRs where ligand binding occurs?
- For some receptors the ligand binding site is formed by (2-3 of) the transmembrane (TM) domains (binding pocket is between the domains of the receptor.
- In other cases the N-terminal region (and other extracellular domains) form the ligand binding site (e.g. peptides, polypeptides and glutamate.
How do GPCRs respond to ligands?
By changing conformation sufficient to attract another protein (G-protein)
How do GPCRs elicit an intracellular response?
- An activated GPCR must interact with another protein called a guaninenucleotide binding protein (G protein)
- G proteins are made up of three subunits ( they are „heterotrimeric‟): α (alpha), β (beta) and γ (gamma) (beta and gamma subunits are functionally dimeric (stay together)
- The GPCR-G protein interaction activates the G protein by causing GTP to be bound instead of GDP on the G protein α subunit
- The α-βγ complex immediately dissociates (into α-GTP + free βγ subunits) and each can then interact with effector proteins (second messenger-generating enzymes, or ion channels)
- 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 heterotrimeric complex.
Why does the alpha G-protein subunit dissociate from the beta-gamma subunit during activation?
When GTP is bound to the alpha subunit, it loses its affinity for the beta-gamma subunit.
What makes the G-protein subunits reassemble after activation?
After activation, the alpha subunit hydrolyses the bound GTP to GDP with its intrinsic enzymatic activity.
The GDP bound alpha subunit regains its affinity for the beta-gamma subunit. This terminates its actions on downstream effectors.
How is the length of a GPCR signal regulated?
GTPase activity on the alpha subunit can be regulated. Timer function controls the gain of the signal (how long the signal lasts)
What gives rise to the diversity of GPCRs?
The human genome encodes 20 Gα (alpha), 5 Gβ (beta)
and 12+ Gγ (gamma) proteins
Therefore, there are >1000 possible Gα-βγ protein combinations
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, Gα subunits and Gβγ subunits interact with specific effector proteins.
How does Receptor-G protein selection give rise to specificity?
It means that an extracellular signal, working via a specific
GPCR, will activate a single, or small sub-population of G
proteins and effectors in the cell to bring about a
specific cellular response.
Give specific examples of GPCRs, their stimulus, G-proteins and their response.
- adrenaline/ noradrenaline- β-adrenoceptor: Gsα, (Gβγ) (upregulates) adenylyl cyclase
- adrenaline/noradrenaline- α2-adrenoceptor: Giα, (Gβγ) (downregulates) adenylyl cyclase
- adrenaline/ noradrenaline- α1-adrenoceptor: Gqα, (Gβγ) (upregulates) phospholipase C
- light- rhodopsin: Gtα (transducin): (upregulates) cyclic GMP and phosphodiesterase
- acetylcholine- M2/M4 muscarinic receptor: Giα, (Gβγ) (downregulates) adenylyl cyclase
- acetylcholine M1/M3 muscarinic receptor: Gqα, (Gβγ) (upregulates) phospholipase C
