theme 2- CB11 Intracellular signalling pathways, diversity of intra-cellular signalling pathways and networks, their regulation and the consequence of their activation

Question 1

What class of extracellular signal molecule is cortisol?

Answer 1

small and hydrophobic

Question 2

Describe the action of a steroid after it has diffused across the plasma membrane e.g. cytosol (steroid signalling)?

Answer 2

1) binds to receptor in nucleus or cytoplasm called cytosolic and nuclear receptor
2) Conformation change = activation of protein
3) promotes or inhibits transcription of a specific target gene- each receptor may act at a different set of regulatory sites in DNA i.e. there can be many target genes

Question 3

Why is steroid signalling essential?

Answer 3

Has a role in human physiology and development, e.g. lack of testosterone receptor, results in female with XY, showing receptor mediates different effects in many cell types

Question 4

what is activated by binding of extra-cellular signal to a cell surface receptor?

Answer 4

intra-cellular signalling pathways

Question 5

Functions of intra-cellular signalling pathways:

Answer 5

1. Relay and transduce the signal
from the point of reception to the
point where a response is
produced
2. Amplifying the signal: a few
extracellular signals elicit a large
intracellular response
3. Distribution of the signal to
different intracellular targets
4. Integrate signals from other
signalling pathways
5. Permit modulation so effects of
the signal can be regulated (positive and negative feedback loops)

Question 6

What are the 2 classes of protein that are molecular switches?
describe switch on and switch off

Answer 6

1) switch proteins controlled by phosphorylation
switch on: activated by phosphorylation by kinase, ATP to ADP
switch off; repressed by dephosphorylation by  protein phosphatase, Pi released
2)GTP-binding proteins
switch on: activated by GTP binding, 
switch off (by themselves): have intrinsic GTPase , so converts GTP to GDP= repressed

Question 7

Name 2 hormones that bind to intra-cellular receptors?

Answer 7

1) steroid hormones

2) Thyroid hormone signalling

Question 8

Name 3 extracellular receptors and where they are found:

Answer 8

1) Ion-channel linked receptors (excitable cells)
2) G-protein linked receptors (most cells)
3) Enzyme-linked receptors (most cells)

Question 9

Where are ion Channel linked receptors

Answer 9

Excitable cells; Muscles, nerves, cardiac cells

Question 10

What do ion-channel-linked receptors do?

Why are they important to excitable cells

Answer 10

change the permeability of the membrane to selected ions =
altering membrane potential = produce current
OR
in the case of Ca2+ ion= regulation of protein activity

Question 11

What does GPCR stand for?

where is it present?

Answer 11

G-protein coupled receptors
most cells (700 cell types)

Question 12

What binds to GPCR?

Describe its structure:

Answer 12

1) a wide range of signalling molecules

2) 7-pass transmembrane structure

Question 13

How does GPCR work?

Answer 13

When extracellular signal molecule binds to receptor of the GPCR it signals to a G-protein,
The G-protein then switches on/off a enzyme/ion channel by travelling to it along membrane and binding.

Question 14

How does the cholera toxin released by cholera interact with the GPCR?

Answer 14

Cholera can activate GPCR by making a permanent change in conformation to the G-protein so always active and transmitting signal to target cells.

Question 15

Describe the processes that occurs including GPCR activation during a rapid “fight-flight” response:
in skeletal muscle cells by adrenaline

Answer 15

1) binding to adrenergic receptor by adrenaline = activated GPCR
2) G-protein moves along membrane and activates adenylyl cyclase (the enzyme)
3) Adenylyl cyclase forms cyclic AMP from ATP, (2nd messenger)
4) cAMP activates PKA (protein kinase) which activates an enzyme that promotes glycogen breakdown.

Question 16

Why is GPCR in rapid “fight-flight” response, rapid?

Answer 16

direct effects on enzyme activity: stimulation of glycogen breakdown in skeletal muscle cells by adrenaline,
effecting enzyme activity doesn’t take as long as synthesis of a protein.

Question 17

What are the effects of adrenaline on the following organs / tissues:

1) Heart
2) skeletal muscles
3) fat tissues

Answer 17

1) increase in heart rate and force of contraction
2) glycogen breakdown
3) fat breakdown

Question 18

Describe the processes that occurs including GPCR activation during a slower response to adrenaline in the brain:

Answer 18

1) binding to adrenergic receptor (GPCR) by adrenaline = activated GPCR/adrenergic receptor
2) G-protein moves along membrane and activates adenylyl cyclase (the enzyme)
3) Adenylyl cyclase forms cyclic AMP from ATP, (2nd messenger)
4) cAMP activates PKA (protein kinase) which diffuses into nucleus and phosphorylates TF
5) Now phosphorylated TF will stimulate transcription of sets of target genes

Question 19

What are the effects of phosphorylation of TF in dues to activation of GPCRs

1) the brain
2) endocrine cells

Answer 19

activation of a set of target genes which results in

1) synthesis of proteins involved in long term memory
2) hormone synthesis

Question 20

What are the 3 effects of GPCR in general:

Answer 20

1) Activation of membrane bound enzymes
2) Activation of phospholipase C
3) Regulation of ion channes

Question 21

How is phospholipase C activated by GPCR and what are its effects?

Answer 21

The G protein activates Phospholipase C,
The enzyme generates small messenger molecules which are inositol triphosphate (IP3) and diacylglycerol (DCG). These molecules leads to changes in intracellular Ca2+ (so in turn activates other molecules e.g. Kinase C

Question 22

What does IP3 do?

Answer 22

inositol triphosphate opens Ca++ channels in ER membrane, release of Ca++ into cytosol, leading to activation of a range of Ca++ responsive proteins (e.g. calmodulin) which go on to activate more proteins (e.g. CaM-kinases)

Question 23

What do DAC and Ca++ being in cytosol do in combination together?

Answer 23

activate protein kinase C (PKC)

Question 24

What do PKA, PKC and CaM-kinases do?

Answer 24

phosphorylate selected target proteins on serines and threonines = altering protein activity

Question 25

How does the activation of GPCR allow for regulation (modulation ) of ion channels:

Answer 25

e.g. acetylcholine binds to GPCR, activating G-protein which moves long membrane and binds to K ion channels activating it= a lower heart rate. (as K+ moves out of cell membrane?)
This is also an example showing that signalling is integrated and regulated with one another; as Electrical impulses can also open K channels????

Question 26

Describe the structure of enzyme-coupled receptors:
SIMILARITIES WITH GPCR
DISSIMILARITIES

Answer 26

S:a transmembrane proteins that display ligand-binding domains on the outer surface of the plasma membrane,
D: not associated with a G protein, the cytoplasmic domain of the receptor either
acts as an enzyme
OR forms a complex with another protein that acts as an enzyme

Question 27

What are the 2 extracellular proteins that enzyme-coupled receptors bind to?
Describe the way the ECR responds and its speed

Answer 27

1) diffusible proteins, local mediators i.e. growth factors : control growth, proliferation, differentiation and survival of cell
-slow as require many intracellular steps that eventually leads to a change in gene expression
2) proteins attached to the surfaces over which a cell is crawling, contact-dependent signal molecules i.e.delta
rapid, reconfiguration of the cytoskeleton

Question 28

Abnormalities in ECR pathways are related to ___________. If __ is constantly activated due to not being able to _____________ which leads uncontrolled cell proliferation resulting in ________.

Answer 28

Oncogenesis ( development of a tumour or tumours)
If Ras is constantly activated due to not being able to hydrolyse GTP which leads uncontrolled cell proliferation resulting in cancer.

Question 29

What is Ras?

Answer 29

small GTP-binding protein that can bind the phosphotyrosines on RTKs that act as docking sites

Question 30

What does Ras do?

Answer 30

activates a three-protein MAP-Kinase signalling module (basically a bunch of 3 MAP-kinases that activate one another the first is called MAP-kinase kinase kinase, the 2nd has 2 kinases in its the name the 3rd one.) that helps relay signal from plasma membrane to the nucleus, which phosphorylates more proteins including transcription regulators = change in gene expression and protein activity= changes in cell behaviour including proliferation and differentiation

Question 31

How does Ras mutations effect RTK signalling pathways?

Answer 31

Ras with a mutation that means it clings to GDP, cannot be activated, but it can still binds to other signalling pathway molecules in the pathway, creating a jam, preventing non-mutant Ras from doing its job = no cell proliferation

Question 32

What is RTK?

Answer 32

receptor tyrosine kinases. are receptors with a cytoplasmic domain that functions as a tyrosine protein kinase which phosphorylate it’s self and selected intracellular signalling proteins on tyrosines.

Question 33

How complex is signalling?

Answer 33

very, a process is activated by multiple receptors, which involves a network of pathways that integrate with each other, it doesn’t just activate an enzyme or TF, but activate and repress multiple cellular functions including assembly of protein complexes.