Session 7 ILOs - Signalling Pathways

Question 1

Understand how signalling molecules are recognised by cells to evoke appropriate cellular responses in G-proteins

Answer 1

Each receptor subtype is specific for one (or limited number of) chemical = ligand (signalling molecule)

In G-proteins, ligands can bind to 2 sites

1. For some receptors, the ligand binding site is formed by 2-3 transmembrane domains
2. In other receptors, the N-terminal region form the ligand binding site

Question 2

Describe how agonist-activated G protein-coupled receptors initiate a sequence of signalling events involving G-proteins and enzyme or ion channel effector proteins

Answer 2

Binding of a ligand to the receptor induces a conformational change, making it able to interact with the G-protein. The alpha subunit undergoes another conformational change, leading to the release of GDP and the binding of GTP. The activated subunit dissociates from the beta-gamma subunit. The activated subunits can then go on to activate/inhibit a membrane bound organelle (adenylyl cyclase or phospholipase C) and produce/inhibit a second messenger.

Question 3

Explain how Pertussis (whooping cough) interacts with the G-protein to cause negative effects

Answer 3

Pertussis toxin covalently modifies the (Gi) inhibitory alpha subunit so that it can no longer exchange ADP for ATP and therefore the G-protein can no longer be inactivated - in the epithelial cells of the airway lining. A persistent cough emerges that can cause death.

Question 4

Explain how Cholera interacts with the G-protein to cause negative effects

Answer 4

Cholera toxin prevents termination of signalling by preventing the GTPase from working so GTP can no longer be hydrolysed by GTPase and so the receptor remains activated = efflux of Cl- ions in the lumen of the small intestine leading to cholera symptoms

Question 5

Understand how activation or inhibition of effector proteins can generate specific cellular responses

Answer 5

Activation or inhibition of the effector proteins can lead to a large increase or decrease in the production of second messengers, which then can impact on other functions of the cell to up regulate cell responses or down regulate cell reponses

Question 6

State the importance of second messengers in signalling pathways and an example of a common second messenger

Answer 6

Second messengers are molecules that relay signals received at G-protein coupled receptors on the cell surface. But in addition to their role as relay molecules, second messengers serve to greatly amplify the strength of the signal, so that you only need a small amount of ligand to produce a physiological response.

EXAMPLE: Calcium ions are one type of second messengers and are responsible for many important physiological functions including muscle contraction, fertilization, and neurotransmitter release

Question 7

Provide 3 clinically-relevant examples of signalling pathways in action

Answer 7

1. Inotropism in the heart
   Adrenaline/noradrenaline binds to the Gs receptor (Beta-1 adrenoceptors), which activates Adenylyl Cyclase to produce cAMP, which then activates cAMP dependent protein kinase, which phosphorylates the voltage gated calcium channels to increase calcium concentration and increase contraction
2. Smooth Muscle contraction
   Noradrenaline (sympathetically released) can bind to the Gq receptor (alpha-1 adrenoceptors) and activate Phospholipase C which then causes PIP2 to release IP3 which binds to the IP3 receptor and releases calcium. Also DAG is produced which binds to the Protein kinase C receptor to because vasoconstriction along with the rise in calcium
3. Neurotransmitter release
   u-opioid receptors are expressed in close proximity to the calcium voltage-gated ion channels. u-opioid receptors (Gi) are activated by Morphine and it’s the beta-gamma subunit that acts on the voltage-gated ion channels to reduce calcium influx and reduces neurotransmitter release