Signal transduction

Question 1

Why is signal transduction important?

Answer 1

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.

Question 2

Give 2 examples of intracellular receptors.

Answer 2

Steroid hormone receptors, thyroid hormone receptors

Question 3

Why are most receptors located on the cellular surface?

Answer 3

The majority of extracellular signalling molecules do not readily cross the plasma membrane

Question 4

Name 3 extracellular signalling molecules.

Answer 4

Hormones, neurotransmitters, growth factors

Question 5

What needs to be present in order for cells to respond to extracellular signals?

Answer 5

The appropriate receptor

Question 6

What are the 3 steps in signal transduction?

Answer 6

Reception
Transduction
Response

Question 7

What are the 3 superfamilies of cell surface receptor?

Answer 7

• 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)

Question 8

What does ligand binding do to a receptor?

Answer 8

Activates the receptor, which in turn directly or indirectly

brings about a change in cellular activity

Question 9

How do ligand gated ion channels work?

Answer 9

Ligand binding „gates‟ the channel to allow ions to move into or out of the cell

Question 10

How do receptors with intrinsic enzymatic activity work?

Answer 10

Ligand binding activates an enzyme activity (e.g. tyrosine

kinase) that phosphorylates the receptor itself + other substrates

Question 11

Describe the insulin receptor.

Answer 11

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

Question 12

Why, clinically, is it important to know about how GPCRs work?

Answer 12

Currently ~40% of all available prescription drugs exert their therapeutic effects directly (as agonists or antagonists) or indirectly at GPCRs

Question 13

What does an agonist do to a GPCR?

Answer 13

Bind to the receptor and activate it

leading to intracellular signal transduction events

Question 14

What does an antagonist do to a GPCR?

Answer 14

Bind to the receptor but do not activate it

block the effects of agonists at the receptor

Question 15

How are GPCR agonists used therapeutically?

Answer 15

Anti-asthma:
β2 adrenoceptor agonists
SALBUTAMOL, SALMETEROL
Analgesia/anaesthesia:
μ-opioid receptor agonists
MORPHINE, FENTANYL

Question 16

How are GPCR antagonists used therapeutically?

Answer 16

Cardiovascular (e.g. hypertension):
β adrenoceptor antagonists
PROPRANOLOL, ATENOLOL
Neuroleptics (e.g. anti-schizophrenic):
D2 dopamine receptor antagonists
HALOPERIDOL, SULPIRIDE

Question 17

What is clopidogrel?

Answer 17

An anti platelet drug (newish)
Used for coronary artery disease/ MI prevention
Irreversible antagonist at PY12 purinoceptor
Warnings being flagged- may have some problems

Question 18

What is aripiprazole?

Answer 18

Anti schizophrenic (atypical). Also used in unipolar and bipolar depression
D2Rs (partial agonist- weak agonistic action)

Question 19

What are the therapeutic applications of drugs targeting GPCRs in the CNS?

Answer 19

Depression 
Schizophrenia 
Psychosis 
Parkinson‟s disease
Migraine

Question 20

What are the therapeutic applications of drugs targeting GPCRs in the CVS?

Answer 20

Hypertension
Congestive heart failure
Cardiac arrhythmia
Thrombosis

Question 21

What are the therapeutic applications of drugs targeting GPCRs in the respiratory system?

Answer 21

Asthma

Chronic obstructive pulmonary disease (COPD)

Question 22

What are the therapeutic applications of drugs targeting GPCRs in the gastrointestinal system?

Answer 22

Acid reflux
Gastric ulcer
Nausea

Question 23

What are the therapeutic applications of drugs targeting GPCRs in the genitourinary system?

Answer 23

Overactive bladder
Prostate cancer
Benign prostatic hyperplasia

Question 24

What are other general therapeutic applications of drugs targeting GPCRs?

Answer 24

Chronic pain
Glaucoma
Rhinitis
Motion sickness
Anaphylaxis

Question 25

What do mutations to G protein-coupled receptors result in? Give 3 examples.

Answer 25

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

Question 26

What are the different stimuli that GPCRs can respond to?

Answer 26

-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))

Question 27

How many different GPCRs have been identified in the human genome?

Answer 27

Over 800 ( in lecture he said around 865). This accounts for over 2% identified genes.

Question 28

What is the basic structure of all GPCRs?

Answer 28

* Single polypeptide chain (300-1200 amino acids) * 7-transmembrane (7TM)- spanning regions * Extracellular N-terminal * Intracellular C-terminal

Question 29

Where are the 2 regions in GPCRs where ligand binding occurs?

Answer 29

* 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.

Question 30

How do GPCRs respond to ligands?

Answer 30

By changing conformation sufficient to attract another protein (G-protein)

Question 31

How do GPCRs elicit an intracellular response?

Answer 31

* 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.

Question 32

Why does the alpha G-protein subunit dissociate from the beta-gamma subunit during activation?

Answer 32

When GTP is bound to the alpha subunit, it loses its affinity for the beta-gamma subunit.

Question 33

What makes the G-protein subunits reassemble after activation?

Answer 33

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.

Question 34

How is the length of a GPCR signal regulated?

Answer 34

GTPase activity on the alpha subunit can be regulated. Timer function controls the gain of the signal (how long the signal lasts)

Question 35

What gives rise to the diversity of GPCRs?

Answer 35

The human genome encodes 20 Gα (alpha), 5 Gβ (beta) and 12+ Gγ (gamma) proteins Therefore, there are >1000 possible Gα-βγ protein combinations

Question 36

What governs Receptor-G protein selection?

Answer 36

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.

Question 37

How does Receptor-G protein selection give rise to specificity?

Answer 37

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.

Question 38

Give specific examples of GPCRs, their stimulus, G-proteins and their response.

Answer 38

- 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