Block D Lecture 1 - Receptor Signalling

Question 1

Where are receptors usually located?

Answer 1

On the external plasma membrane (with the exception of nuclear receptors)

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Question 2

What are 4 different types of receptors?

Answer 2

Ligand-gated ion channel receptors
G-protein coupled receptors (GPCRs)
Kinase-linked receptors
Nuclear receptors

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Question 3

What are 2 alternative names for GPCRs?

Answer 3

Metabotropic or 7TM receptors

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Question 4

What is the basic structure of a GPCR?

Answer 4

It comprised of 7 α-helices which are transmembrane, forming loops. One of the intracellular loops is larger than the others and is responsible for interacting with the G-protein

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Question 5

What 3 subunits are G proteins made of?

Answer 5

α, β and γ

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Question 6

How do G proteins become activated?

Answer 6

1. In their inactive state, the α subunit of the G protein is bound to GTP
2. When a ligand binds to a GPCR, it changes conformation which results in the receptor promoting the exchange of GDP for GTP
3. The binding of GTP activates the G protein and causes the α subunit to dissociate from the βγ dimer
4. It is this α subunit which interacts with the effector

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Question 7

How are cAMP and protein kinase A involved in the breakdown of glycogen?

Answer 7

1. cAMP activates protein kinase A (PKA)

2a. In the liver, PKA catalyses the phosphorylation of glycogen synthase, inactivating it

2b. At the same time as the above, PKA phosphorylates phosphorylase kinase and activates it

3. The activated phosphorylase kinase phosphorylates phosphorylase B and activates it, turning it into phosphorylase A
4. Phosphorylase A causes glycogen to be broken down into glucose 1-phosphate

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Question 8

How are muscarinic M3 receptors and protein kinase C involved in contraction of the airways?

Answer 8

As acetylcholine binding to the M3 receptor leads to the Gq protein being activated, which eventually ends up cleaving PIP2 into IP3 and DAG, with DAG activating protein kinase c and IP3 releasing calcium, their effects combining to ensure airway contaction

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Question 9

Are there any inhibitors of PKA or PKC use clinically?

Answer 9

No
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Question 10

What is the purpose of creating antibodies for GPCRs?

Answer 10

They can be used for research to study GPCRs or identify conformational changes. They can also be used therapeutically to modulate GPCR activity like agonists and antagonists or they can block ligand binding

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Question 11

What are pepducins?

Answer 11

A class of cell-penetrated lipidated peptides

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Question 12

What is the basic structure of pepducins (and lipopeptides in general)?

Answer 12

They are comprised of 2 parts:

1. A synthetic peptide which is a 7-20 length amino acid fragment of a GPCR intracellular loop and interacts with its related receptor.
2. A hydrophobic moiety (meaning divided into 2 parts) which is cell-penetrating

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Question 13

What is the function of pepducins?

Answer 13

They flip across the cell membrane and are directed at intracellular loop domains which allow GPCRs to couple to intracellular G proteins, preventing the receptor from coupling to the G protein

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Question 14

What does the PZ-235 pepducin currently in phase 1 clinical trials supposed to be able to treat?

Answer 14

Kidney fibrosis, idiopathic pulmonary fibrosis and non-alcoholic steatohepatitis (a non alcoholic fatty liver disease)

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Question 15

What are β-arrestins involved in?

Answer 15

GPCR receptor desensitisation and internalisation

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Question 16

How can GPCRs activate proteins (such as ERK) without the use of G proteins?

Answer 16

By using β-arrestin. It can bind to a lot of other things and can even provide a scaffold for activation of other signalling pathways

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