Intracellular Signalling - 2nd Messengers

Question 1

What are 2nd messengers?

Answer 1

Short-acting intracellular molecules that are rapidly formed or released due to receptor activation

Question 2

What are the 5 common 2nd messengers?

Answer 2

1. cAMP
2. cGMP
3. Diacylglycerol (DAG)
4. Inositol 1,4,5-triphosphate (IP3)
5. Intracellular calcium (Ca2+)

Question 3

How are most 2nd messengers formed?

Answer 3

From other molecules by effector enzymes

Question 4

How is cAMP/cGMP/DAG/IP3 formed?

Answer 4

1. ATP –> cAMP
2. GTP –> cGMP
3. PIP2 –> DAG + IP3

Question 5

How is intracellular calcium formed?

Answer 5

Not ‘formed’ but released into cytosol from intracellular stores (endoplasmic reticulum) or enters cell from extracellular sources (via calcium channels)

Question 6

What is production of cAMP from ATP catalysed by?

Answer 6

Adenylate cyclase (adenylyl cyclase)

Question 7

What is production of cGMP from GTP catalysed by?

Answer 7

Guanylate cyclase (guanylyl cyclase)

Question 8

What are 2 ways that cGMP can be produced?

Answer 8

1. Activation of soluble guanylate cyclase by NO

2. Activation of membrane-bound guanylate cyclase in response to e.g. neuropeptides (ANP)

Question 9

What is cAMP broken down by and to?

Answer 9

By specific phosphodiesterases (PDEs) to form AMP

Question 10

What is cGMP broken down by and to?

Answer 10

By specific phosphodiesterases (PDEs) to form GMP

Question 11

What is importance of PDEs?

Answer 11

Switching off signalling pathway by reducing levels of cAMP and cGMP and reducing response

Question 12

What are 2 PDE inhibitors?

Answer 12

Caffeine and viagra

Question 13

What are DAG and IP3 produced as a result of?

Answer 13

Activation of the G-protein Gq

Question 14

What happens when GTP-bound Gq stimulates membrane-localised phospholipase C (PLC)?

Answer 14

Catalyses production of the 2nd messengers DAG and IP3 from the membrane lipid PIP2

Question 15

What is effect of IP3?

Answer 15

Polar molecule that can diffuse through cytosol to the ER where it interacts with calcium channels to stimulate the release of stored Ca2+ ions into the cytosol

Question 16

What is effect of DAG?

Answer 16

Remains in the membrane and stimulates protein kinase C (PKC) that phosphorylates target proteins leading to cellular response

Question 17

What else can PKC be activated by?

Answer 17

Ca2+ released from ER

Question 18

Where is calcium stored under normal conditions?

Answer 18

In the ER and mitochondria –> so the cytosolic concentration is low

Question 19

What happens when receptor is activated and IP3 binds to receptors in ER?

Answer 19

Efflux of Ca2+ from ER

Ca2+ then acts as 2nd messenger

Question 20

How is Ca2+ taken back up into the ER?

Answer 20

Through a calcium ATPase in the ER membrane (or is pumped or exchanged out of cell)

Question 21

What are example receptors that activate the Gq activation pathway (producing DAG, IP3 and Ca2+)?

Answer 21

angiotensin II acting on AT1 receptor, adrenaline acting on alpha1-adrenergic receptor

Question 22

Describe alpha subunit of Gq when inactive

Answer 22

Alpha subunit of Gq is bound to GDP and is therefore in the “OFF” state

Phospholipase C is also inactive as has not been activated by G-protein

Question 23

What is Phospholipase C?

Answer 23

A family of eukaryotic intracellular enzymes that play an important role in signal transduction processes

Question 24

What does ligand binding to G receptor in Gq activation pathway cause?

Answer 24

1. Causes receptor to associate with G protein (Gq)
2. This stimulates displacement of GDP by GTP (switching G-protein “on”) and alpha-subunit dissociates from beta/gamma subunits

Question 25

Describe receptor, G-rptoein and effector enzyme involved in the Gq activation pathway (production of DAG, IP3 and Ca2+)

Answer 25

1. Receptor –> GPCR
2. G-protein –> Gq
3. Effector enzyme –> phospholipase C

Question 26

How is effector enzyme and 2nd messenger production stimulated in Gq pathway?

Answer 26

GTP-bound Gq stimulates membrane-localised phospholipase C (PLC) which catalyses production of two different second messengers DAG and IP3 from the membrane lipid PIP2

Question 27

What is IP3? How can it stimulate release of Ca2+?

Answer 27

IP3 is a polar molecule that can diffuse through the cytosol to the ER where it interacts with calcium channels to stimulate release of stored Ca2+ ions into the cytosol

Question 28

What is effect of increase in intracellular Ca2+?

Answer 28

Stimulates various cellular processes

Question 29

What is effect of DAG generated?

Answer 29

Remains in the membrane and stimulates protein kinase C (PKC) that phosphorylates target proteins leading to cellular responses

Question 30

What else can PKC be activated by?

Answer 30

Ca2+ released from the ER

Question 31

Ca2+ is not “formed” like the other second messengers. How is it released?

Answer 31

Released into the cytosol from intracellular stores (e.g. endoplasmic reticulum) or enters the cell from extracellular sources (via calcium channels)

Ca then acts as a second messenger to activate various molecules (e.g calcium-dependent kinases) to modulate cellular function

Question 32

Why, under normal conditions, is cytosolic Ca2+ concentration low?

Answer 32

Calcium is isolated in the ER and mitochondria under normal conditions

Question 33

What can increase in intracellular Ca2+ trigger?

Answer 33

Can trigger opening of calcium channels in the plasma membrane so that even more Ca floods into the cell

Question 34

How is Ca2+ taken back up into the ER?

Answer 34

Through a calcium ATPase in the ER membrane, or is pumped or exchanged out of the cell