Intracellular Signalling - 2nd Messengers Flashcards
What are 2nd messengers?
Short-acting intracellular molecules that are rapidly formed or released due to receptor activation
What are the 5 common 2nd messengers?
- cAMP
- cGMP
- Diacylglycerol (DAG)
- Inositol 1,4,5-triphosphate (IP3)
- Intracellular calcium (Ca2+)
How are most 2nd messengers formed?
From other molecules by effector enzymes
How is cAMP/cGMP/DAG/IP3 formed?
- ATP –> cAMP
- GTP –> cGMP
- PIP2 –> DAG + IP3
How is intracellular calcium formed?
Not ‘formed’ but released into cytosol from intracellular stores (endoplasmic reticulum) or enters cell from extracellular sources (via calcium channels)
What is production of cAMP from ATP catalysed by?
Adenylate cyclase (adenylyl cyclase)
What is production of cGMP from GTP catalysed by?
Guanylate cyclase (guanylyl cyclase)
What are 2 ways that cGMP can be produced?
- Activation of soluble guanylate cyclase by NO
2. Activation of membrane-bound guanylate cyclase in response to e.g. neuropeptides (ANP)
What is cAMP broken down by and to?
By specific phosphodiesterases (PDEs) to form AMP
What is cGMP broken down by and to?
By specific phosphodiesterases (PDEs) to form GMP
What is importance of PDEs?
Switching off signalling pathway by reducing levels of cAMP and cGMP and reducing response
What are 2 PDE inhibitors?
Caffeine and viagra
What are DAG and IP3 produced as a result of?
Activation of the G-protein Gq
What happens when GTP-bound Gq stimulates membrane-localised phospholipase C (PLC)?
Catalyses production of the 2nd messengers DAG and IP3 from the membrane lipid PIP2
What is effect of IP3?
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
What is effect of DAG?
Remains in the membrane and stimulates protein kinase C (PKC) that phosphorylates target proteins leading to cellular response
What else can PKC be activated by?
Ca2+ released from ER
Where is calcium stored under normal conditions?
In the ER and mitochondria –> so the cytosolic concentration is low
What happens when receptor is activated and IP3 binds to receptors in ER?
Efflux of Ca2+ from ER
Ca2+ then acts as 2nd messenger
How is Ca2+ taken back up into the ER?
Through a calcium ATPase in the ER membrane (or is pumped or exchanged out of cell)
What are example receptors that activate the Gq activation pathway (producing DAG, IP3 and Ca2+)?
angiotensin II acting on AT1 receptor, adrenaline acting on alpha1-adrenergic receptor
Describe alpha subunit of Gq when inactive
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
What is Phospholipase C?
A family of eukaryotic intracellular enzymes that play an important role in signal transduction processes
What does ligand binding to G receptor in Gq activation pathway cause?
- Causes receptor to associate with G protein (Gq)
- This stimulates displacement of GDP by GTP (switching G-protein “on”) and alpha-subunit dissociates from beta/gamma subunits
Describe receptor, G-rptoein and effector enzyme involved in the Gq activation pathway (production of DAG, IP3 and Ca2+)
- Receptor –> GPCR
- G-protein –> Gq
- Effector enzyme –> phospholipase C
How is effector enzyme and 2nd messenger production stimulated in Gq pathway?
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
What is IP3? How can it stimulate release of Ca2+?
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
What is effect of increase in intracellular Ca2+?
Stimulates various cellular processes
What is effect of DAG generated?
Remains in the membrane and stimulates protein kinase C (PKC) that phosphorylates target proteins leading to cellular responses
What else can PKC be activated by?
Ca2+ released from the ER
Ca2+ is not “formed” like the other second messengers. How is it released?
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
Why, under normal conditions, is cytosolic Ca2+ concentration low?
Calcium is isolated in the ER and mitochondria under normal conditions
What can increase in intracellular Ca2+ trigger?
Can trigger opening of calcium channels in the plasma membrane so that even more Ca floods into the cell
How is Ca2+ taken back up into the ER?
Through a calcium ATPase in the ER membrane, or is pumped or exchanged out of the cell
