Intracellular Signalling Flashcards
What is the purpose of signal transduction?
To convert an extracellular signal into a cellular response
What are the 3 main stages of signal transduction?
- reception
- signal transduction
- cellular response
What is meant by ‘heirarchy’ in intracellular signalling?
It describes how the components of a signal transduction pathway are arranged in a specific order to transmit a signal from the outside of a cell to the inside of the cell
What are the components involved in a hierarchy?
- first messenger
- receptor
- G-protein
- effector enzyme
- second messenger
- protein kinase
- target protein
- cellular response
What is meant by amplification in a signal transduction pathway?
The signal transduction pathway amplifies the initial signal
A single first messenger molecule can induce many downstream signalling molecules
This leads to a larger cellular response
How is G-protein activation involved in amplification?
A single G-protein molecule can activate many molecules of effector enzyme
How is the effector enzyme involved in amplification?
The effector enzyme will catalyse many reactions without being used up
This leads to the production of many molecules of second messenger
How is the protein kinase involved in amplification?
The protein kinase will catalyse the phosphorylation of many molecules of protein substrate
What is meant by the specificity of signal transduction pathways?
Signal transduction pathways are highly specific
The first messenger may bind to a single receptor to elicit a single response
Can one signalling molecule work on different types of cells?
The first messenger must act on the SAME receptor
It can stimulate different responses in different cells due to the differential expression of signalling components
What happens if a signal transduction pathway branches?
This leads to more than one cellular response
What is meant by “cross-talk” between two signalling pathways in the same cell?
First messengers that bind to different receptors on the same cell may modulate the cellular response
They may activate or inhibit the original signal transduction pathway
What happens if the same first messenger acts on a different type of receptor?
This will stimulate a different signal transduction pathway
This leads to a very different cellular response
How does adrenaline act to cause different actions in:
i. muscle and liver
ii. adipose tissue
iii. heart
iv. blood vessels
i. stimulates breakdown of glycogen
ii. stimulates fatty acid production
iii. increases the heart rate by stimulating contraction of cardiomyocytes
iv. increases blood pressure by causing relaxation of vascular smooth muscle cells
What are G-proteins and what do they bind?
Guanine nucleotide binding proteins
They bind GTP and GDP
(guanosine tri/diphosphate)
What is the role of GTP?
It is a high energy molecule that activates G-proteins
What is meant by G-proteins being GTPase enzymes?
They catalyse the hydrolysis of GTP to GDP
GDP switches off the G-protein
How are G-proteins anchored to the internal surface of the cell membrane?
By lipid tails via prenylation
These tails are either farnesyl or geranylgeranyl groups
What are the 2 major groups of G-proteins?
- heterotrimeric receptor-associated G-proteins
2. small GTPases
What activates a heterotrimeric receptor associated G-protein?
Why are they heterotrimeric?
They are activated by GPCRs
They are heterotrimeric as they contain 3 different subunits: alpha, beta, gamma
How do the subunits vary between different classes of heterotrimeric G-proteins?
Why?
They have different alpha subunits but share beta and gamma subunits
The alpha subunits contain the GTPase activity
What is the function and structure of small GTPases?
They are monomeric as they only contain 1 subunit
They are involved in cell signalling, cytoskeletal regulation and vesicle trafficking
When are G-proteins active and inactive?
They are active when bound to GTP
They are inactive when bound to GDP
What happens when the ligand first messenger binds to its receptor (GPCR)?
This induces a conformational change in the receptor allowing the G-protein to bind to the receptor
What happens once the G-protein has bound to the receptor?
This stimulates the G-protein to exchange GDP for GTP
This switches it on and allows it to activate the effector enzymes
The G-protein then hydrolyses GTP back to GDP, switching it off
What is the state of the alpha subunit of Gs before the receptor is activated?
The alpha subunit of Gs is bound to GDP
It is in the inactive state
Adenyl cyclase is inactive as it has not been activated by a G-protein
What happens when the first messenger binds to the GPCR?
The G-protein releases GDP and swaps it for GTP
This switches the G-protein on
The GTP-bound alpha subunit dissociates from the beta and gamma subunits
What happens to the GTP-bound Gs-alpha subunit after it has dissociated?
It binds to and activates adenylyl cyclase
This catalyses the conversion of ATP to cAMP
cAMP is the second messenger
What is the role of the GTPase activity of the Gs-alpha subunit?
It hydrolyses GTP back to GDP to inactivate the G-protein
The GDP-bound alpha subunit reassociates with the beta and gamma subunits
What will break down cyclic AMP?
Phosphodiesterases break down cAMP to AMP
How do different alpha subunits of G-proteins affect different enzymes?
Gs - activates adenyl cyclase to increase cAMP
Gi - inhibits adenyl cyclase to reduce cAMP
Gg - activates phospholipase C to increase DAG and IP3
How do the actions of Gs and Gi oppose one another?
Activating Gs leads to stimulation of adenyl cyclase and increased levels of cAMP
Activating Gi inhibits adenyl cyclase and leads to reduced levels of cAMP
How does the cholera toxin affect G-protein activity?
It prevents GTPase activity of Gs
GTP remains bound to Gs so that it remains in the active state
What is the consequence of the cholera toxin keeping Gs in the active state?
It leads to overstimulation of adenyl cyclase and accumulation of cAMP
What is the result of accumulation of cAMP in intestinal epithelial cells?
Elevated cAMP increases loss of chloride ions through chloride channels
This leads to water being excreted into the intestinal lumen and diarrhoea
What causes whooping cough?
Bordetella pertussis bacteria in airborne respiratory droplets
What is the virulence factor of whooping cough?
Pertussis toxin
How does the pertussis toxin affect G-proteins?
It prevents GDP/GTP exchange by Gi
The Gi protein is locked in the off position and is unable to inhibit adenyl cyclase
What is are the physiological effects caused by the pertussis toxin?
cAMP accumulates leads to increased insulin secretion and increased sensitivity to histamine
What are second messengers?
Short-acting intracellular molecules that are rapidly formed as a result of receptor activation
What are the 5 common second messengers?
- cyclic AMP - cAMP
- cyclic GMP - cGMP
- diacylglycerol - DAG
- inositol 1,4,5-triphosphate - IP3
- intracellular calcium - Ca2+
How are most second messenger molecules formed?
Most second messengers are formed from other molecules by effector enzymes
What is the difference in the way in which intracellular calcium is formed?
It is not formed
It is released into the cytosol from intracellular stores in the ER
How is cyclic AMP produced?
It is produced from ATP via adenylyl cyclase
How is cyclic GMP produced?
It is produced from GTP
via guanylyl cyclase
How may cGMP be produced using guanylyl cyclase?
- activation of soluble guanylate cyclase by nitric oxide
2. activation of membrane-bound guanylate cyclase in response to neuropeptides
Which phosphodiesterases will break down cAMP only?
PDE 4, 7, 8
Which phosphodiesterase will break down cGMP only?
PDE 5, 6, 9
Which phosphodiesterases will break down BOTH cAMP and cGMP?
1, 2, 3, 11, 12
Why are PDEs important?
What are examples of PDE inhibitors?
They reduce the levels of cAMP and cGMP, reducing the response
PDE inhibitors are caffeine and viagra
What will activate the Gq activation pathway?
Angiotensin II acting on the AT1 receptor
Adrenaline acting of the alpha1-adrenergic receptor
What happens to the alpha subunit of Gq before the ligand binds?
It is in the inactive state and is bound to GDP
Phospholipase C is inactive as it has not been activated by the G-protein
What happens when the ligand binds to the receptor?
The receptor associates with Gq
This stimulates the displacement of GDP for GTP and the G-protein is activated
The alpha subunit dissociates from the beta and gamma subunits
What will GTP-bound Gq stimulate?
It will stimulate membrane-localised phospholipase C
What will phospholipase C catalyse once it has been activated?
The production of 2 different second messengers from the membrane phospholipid PIP2
- diacylglycerol - DAG
- inositol-1,4,5-trisphosphate - IP3
What happens to IP3 once it has been produced by phospholipase C?
It diffuses through the cytosol to the ER
It interacts with Ca2+ channels, leading to the release of stored Ca2+ into the cytosol
What happens to DAG once it has been formed by phospholipase C?
It remains in the membrane and stimulates protein kinase C
This will phosphorylate target proteins
How does intracellular calcium act as a second messenger?
It activates various molecules that will modulate cellular function
e.g. calcium dependent kinases
What can an increase in intracellular calcium concentration sometimes trigger?
The opening of calcium channels in the plasma membrane
This causes even more calcium to enter the cells
How is calcium taken back up again?
It is taken back up into the ER through a calcium ATPase in the ER membrane
What are protein kinases?
They are enzymes that facilitate the transfer of a phosphate group from ATP to a specific amino acid residue on a specific protein
What amino acid residues can be phosphorylated by protein kinases?
Why?
- serine
- threonine
- tyrosine
These residues have side chains containing a hydroxyl group where the phosphate group can be added
What is the purpose of phosphorylating proteins?
It changes the function of the protein
It may activate or inhibit protein function
On which part of the protein can phosphorylation occur and why?
It can only occur on intracellular domains of the protein as this is where kinases are found
What is a human phosphoprotein?
A long protein that has multiple phosphorylation sites
It is phosphorylated by several different kinases
What are the 3 main types of protein kinases?
- serine/threonine kinases
- tyrosine kinases
- dual-specificity kinases
What is an example of a dual-specificity kinase?
MAP kinase
They phosphorylate serine, threonine and tyrosine residues
What is the role of a phosphatase?
They remove phosphate groups from amino acid residues
This opposes the effects of kinases
What are the different types of phosphatases?
- serine/threonine-directed phosphoprotein phosphatases
2. tyrosine-directed phosphotyrosine phosphatases
Why are there not as many phosphatases as there are kinases?
Phosphatases have a much broader specificity
What are the 2 ways in which kinases can modulate protein function?
- phosphorylation of a protein
This leads to a conformational change that directly alters the function of the protein
- phosphorylation of a transcription factor
This activates or inhibits transcription of a gene, and protein expression levels
What is the “A pathway” involving adrenaline?
- adrenaline - first messenger
- B1-AR - receptor
- Gs
- Stimulation of adenyl cyclase
- cAMP production
- activation of protein kinase A
What is the “A pathway” involving acetylcholine?
- acetylcholine - first messenger
- muscarinic M2 - receptor
- Gi
- Gi inhibits adenyl cyclase
- inhibition of cAMP production and protein kinase A
What is the “C pathway” in second messenger signalling?
- angiotensin II
- AT1R - receptor
- Gq
- Gq activates phospholipase C
- Phospholipase C activates DAG and IP3
- DAG activates protein kinase C
IP3 activates intracellular calcium release
This leads to activation of protein kinase C and Ca/CaM kinase
What is the “G pathway” in second messenger signalling?
- neuropeptide or NO
- this directly activates guanylate cyclase
- this leads to cGMP production
- cGMP activates protein kinase G
For which diseases are protein kinase inhibitors being developed for use as therapeutic agents?
- cancer
- cardiovascular disease
- HIV/AIDS
- rheumatoid arthritis
- alzheimer’s
Why may protein kinase inhibitors have a use in cancer treatment?
Dysregulation of many kinases is directly linked to cancer development
There are changes in protein kinase expression levels in solid tumours
