Flashcards in Martin Cann - Signalling Deck (71):
What is the effect of extracellular signalling molecules bind to specific receptors?
- it initiates a response within the target cell. Most receptors are found at the plasma membrane.
How can hydrophobic signalling molecules pass into the cell?
- some hydrophobic molecules can pass via the plasma membrane into the cell.
- because they're hydrophobic they must be carried to the target cell by proteins that protect them.
How do signalling molecules bind to the receptor and what is the effect?
- Hormone/receptor complex recruits a second receptor molecule. The receptor is therefore activated through dimerization. Other receptors are activated by conformational changes.
What causes changes in the cell when the signalling molecule are receptor binds?
- the conformational change is what is detected by the inside of the cell
What is the function of endocrine signalling?
- co-ordinates behaviour over long distances.
- specialised cells secrete Luganda called hormones into the blood stream (e.g. Adrenaline) or sap (e.g auxin) to be distributed around the body.
- Slow signalling
What is the function of paracrine signalling?
- Acts over small distances.
- diffusion of ligand is limited by extracellular matrix and enzymes e.g Wnt, Hedgehog, and BMP proteins in cell fate determination in development.
> important in cell fete determination in development
What is the function of autocrine signalling?
- the cell proudces a ligand that binds to its own receptors e.g. An reinforce a developmental decision among cells.
- Mechanism is strongest among groups of cells enabling that group to enter a specific developmental pathway e.g. Insulin-like growth factor in muscle development.
- Eicosenoids are fatty acid derivatives made by cells in all mamillian tissues.
- On tissue damage Eicosenoids production increases and acts in an autocrine fashion to mediate the pain, fever and inflammatory responses.
> cell signals to itself
What is contact dependant signalling for?
- signalling of integral membrane proteins adjecent cell.
> in contrast to paracrine signalling. in paracrine signalling to neighbour it is by diffusion this means that the the signal is sent to more than directly adjecent neighbour.
What happens if a cell receives no signals?
- it will die by apoptosis.
- for a cell to survive it must receive a minimum of compliment molecules. This allows it to survive and grow.
What are 2 factors that determine the response of a cell to a combination of signalling molecules?
1. The subset of receptors that the cell possesses to detect those signals
2. The nature of the intracellular machinery by which the cell interrupt the signal.
What is No produced in response to?
- sympathetic stimulation of endothelial cells adjoining Vascualature smooth muscle cells. NO passes to smooth muscle cells causing them to relax and for blood flow to increase.
Explain smooth muscle realisation via signalling molecules.
- nervous signal comes to synapse, endothelial cell layer, then underneath a smooth muscle cell layer in the blood.
- Acetyl choline is released on activation of sympathetic nervous system. To have the desired effect the signal must get through different cell layers.
- Actyl is released and must
what are G-protein linked receptors? What do they do?
- Largest family of cell surface receptor (7-TM receptors) with a huge variety of signalling ligand a (hormones, neurotransmitters, local mediators).
- Receptors mediate processes as diverse metabolism, learning and memory, olfaction so and visions.
What is the common structures of 7-Tm passes?
- extracellular ligand binding domain, the intracellular G-protein binging domain.
- the ligand for the receptor is known as the 'first messanger'. Thought to have evolved from non-G-protein linked structurally similar receptors in lower organisms.
What happens when G proteins are switches off?
- it binds to Guanidine diphosphate (GDP).
> when a ligand binds to receptor and activates it. The GDP is lost in exchange for GTP.
What are the 6 steps of the cycle involving G-protein receptors?
1. Binding of hormone induces a conformational change in the inactive receptor- activating it.
2. Activated receptor binds to Galpha subunit
3. Activated receptor causes conformational change in Galpha, triggering dissociation of GDP.
4. Binding of GTPto Galpha triggers dissociation of Galpha both from the receptor and from Gbetagamma.
5. Hormone dissociates from receptor, Galpha binds to effector activating it.
6. Hydrolysis of GTP to GDP causes Galpha to dissociate from effector and re-associate with Gbetagamma.
What is the main response of Luteinizing hormone on the ovaries?
- progesterone secretion
What is the major effect of adrenaline on the heart muscle?
- Glycogen breakdown, increase in heart rate "fight or flight".
What is the major response of glucagon on the liver?
- Glygogen breakdown
What is the major response of vasopressin on the kidneys?
- water resorption.
What is cAMP syntheses bY.
Adenyl cyclase (AC).
How do G-proteins regulate AC?
- AC is activated by one type of G protein (Gas) and inhibited by another (Gai).
- AC syntheses cAMP from ATP and cAMP is cAMP-phosphodiesterase to turn off the signal. CAMP is a 'secondary messenger'.
- there are 10 isoforms of AC in mammals, 9 regulated by G-proteins.
- G protein regulated ACs can also be regulated by Gbg and the secondary messanger Ca2+ for added complexity.
What is the effect of different hormones binding to receptors which are either stimulatory or inhibitory for AC?
- end result is that cAMP production is either up regulated or blocked.
What is the effect is G-alpha on g-protein?
- activates it
What is the effect of G-alpha I on adenyl cyclise?
- Switches it off
What is the target for cyclic AMP?
- Protein kinase
WHat is the result of having more molecules of cAMP than the starting hormone?
The extracellular signal is amplified.
What is the effect of cAMP binding and activating PKA?
- cAMP binding releases regulatory subunits from the catalytic subunits therefore activating the ......
What does PKA activate?
- the cAMP responsive element binding protein (CREB).
What does CREB do?
When activated and phosphorylated it beocomes known as a CRE-binding protein (CBP) which binds to a CREB-binding element and can activate specific target genes causing transcription and translation.
What is PI(4,5)P2?
Makes up less than 1% of cell phospholipid and is the target of the G-protein activated enzyme phospholipase C-b. The G-proteins Galpha o / Galpha q activate PLC C-b
How is IP3 produced?
- PIP2 is acted on by phospholipase C whcih produces IP3 due to cleavage of the compound
What happens to IP3?
- it is the head group of IP2. It can leave behind the IP2 tail in the membrane and can signal within the cell.
- it goes to the ER where it binds to its receptor IP3 receptor a calcium channel. The sailor store of calcium. Calcium is a very powerful signalling molecule in its own right.
What happens to the calcium that's produced?
- binds to a kinase
- protein kinase C
What does protokinase C do?
Phosphorylates substrates ....
What are the 3 classes of enzyme linked surface receptor?
- Tyrosine kinases
> receptor tyrosine kinases
> Tyrosine associated receptors
- Receptor serine/ there nine kinases
- Receptor guanylyl cyclases
What is the name of the receptor for epidermal growth factor (EGF) and what is the response?
- receptor = EGF
- stimulates proliferation of various cell types.
What is the effect o insulin?
- stimulates carbohydrate utilisation and protein synthesis
What is the receptor for nerve growth factor (NGF) and what is the effect?
- Trk A
- Stimulates survival and growth of some neurones
What is the structure of tyrosine kinases?
- all tyrosine kinase receptors show common mechanism and action.
- the extracellular domain of the receptor monomer binds to the growth factor ligand.
- A conformational change on ligand binding permits receptor dimerization.
- the two receptors phosphorylated eachother on tyrosine residues in an 'activation lip'.
- phosphorylation of the lip permits full activation and phosphorylation on a range of other tyrosine residues
In 3 steps explain the tyrosine signalling pathway.
1. Receptor tyrosine kinases (RTKs) without bound ligand.
2. Dimerization and phosphorylation of action lip tyrosine so
3. Phosphorylation of additional tyrosine residues.
Explain in more detail the tyrosine kinase signalling pathway.
- auto phosphorylation of the receptor tyrosine kinase initiates a signalling cascade that involves adapted proteins, the small monomeric G-protein Ras, and the MAP protein kinase cascade to transducer the signal to the nucleus. The signal eventually elicits a new round of transcription/ translation to change cell behaviour as dictated by theorigional ligand e.g. Neuronal growth
What are ligand so that act through tyrosine kinase associated receptors?
- cell surface receptors that do not possess intrinsic tyrosine kinase activity.
- A tyrosine kinase encoded by another gene is often non-covalently associated with the receptor (largest family in mammals are the Src family of tyrosine kinases).
An important member of the ligand associated tyrosine kinase receptors include intergrins what do these do?
Cytokines are also very important subfamily
- bind to extra cellular matrix.
- Intergrin activation on matrix binding activates the cytoplasmic tyrosine kinase FAK (focal adhesion kinase) and alters cell-matrix adhesion properties.
What is cytokines signalling though cytokine receptors depends upon?
the JAK family of cytoplasmic tyrosine kinases.
What happens when a ligand binds to a receptor homodimer?
- JAKs phosphorylate each other on tyrosine residues.
What happens to the activated JAKs?
- phosphorylate the receptor on tyrosine residues.
- this allows STAT (signal transducers and activators of transcription) proteins to bind.
What happens after the STATs have bound?
- the STATs proteins are phosphorylated in tern, dimerization, dissociate from the receptor and migrate to the nucleus to initiate transcription.
- the pattern of transcription if determined by the original signal and the cell type.
In 3 step summarise cytokine signalling. With then another. Steps
1. Cytokine receptors without bound ligand a
2. Dimerization and phosphorylation of activation lip tyrosine a
3. Phosphorylation of additional tyrosine residues.
1. STAT is phosphorylated
3. Into nucleus binds DNA and activates transcription
What signalling ligand use serine/threonine kinases?
In what way do the TGF-betas and BMPs act?
- in an endocrine or paracrine fashion.
What are the TGF-betas and BMPs important for?
In development and regulate pattern formation, proliferation, differentiator, extracellular matrix production, cell death and immune function. They singnal though single pass transmembrane proteins with a cytosolic SER/the-kinase domain and transmit the signal to the nucleus to initiate transcription. .
What are the 4 steps involved in receptor serine/threonine kinase signalling?
- TGF-B binding to a type-II receptor causes the receptor to recruit and phosphorylate a type-I receptor
- Phosphorylated Type-I receptor requires and phosphorylated Smad2 or Smad3.
- Phosphorylated Smad2/3 dissociates from the receptor and oligomerizes with Smad4.
- The Smad2/3/4 oligomer migrates to nucleus, recruits other gene regulatory proteins and activates transcription of specific target genes.
>> resutls in target gene transcription
How in single transmembrane pass receptors is cGMP syntheses and what does it do?
- syntheses from GTP.
- CGMP activates protein kinase G.
To what do the Wnt and hedgehog (Hh) receptors bind?
- to the Wnt and Hh family of signalling molecules respectively.
They are muti-transmembrane domain spanning receptors but do not couple to G-proteins.
- Wnt and Hh ligands function primarily in developmental processes.
What is the name given to Wnt receptors?
- 7 transmembrane pass proteins called frizzled (Fz) proteins.
- UNlike 7-TM receptors discussed previously, Fz receptors are not coupled to G-proteins.
What so Wnt effect?
- major role in brain development, limb patterning, organogenesis, control of bone densit and stem cell development.
What are Wnt proteins modified with and how does this effect them?
- modified with palmitate and so are tethered to the plasma membrane of the cell from which they're secreted. Wnt proteins can therefore only mediate contact dependant signals.
How does Wnt have an effect?
- it causes changes in gene transcription on target cels through TCF transcription factor but TCF only activates transcription in the presence of B-care in.
- in the absence of Wnt, B-caritine is maintained in a complex with axis (a scaffold protein), adenomatosis polyposis coli (APC: an important human tutor supresser, and the GSK3 protein kinase.
What is the effect of GSK3?
- it phosphorylated B-catenin and is targeted for degradation in the proteosome.
- as the B-catenin is degraded, it does not bind to TCF and the lone TCF Preston inhibits the transcription of Wnt dependant genes.
What happens after transcription of WNt dependant genes is inhibited?
- Wnt binds to the Fz receptor.
- In the presence of Wnt, Fz is able to interact with its co-receptor (LRP). LRP in the Wnt-Fz-LRP complex is phosphorylated but GSK3 and this permits axin binding.
What is the effect of Axin binding?
Axin binding to phospho-LRP disrupts the GSK3-APC-Axin-Bcatenin complex.
B-catering is therefore no longer phosphorylated by GSK3.
- this results in B-catenin accumulation in the cell (a process that also requires the dishevelled (dsh) protein bound to Fz and translates to the nucleus. B-care in associates with TCF to up regulate the expression of Wnt dependant genes.
What is similar about the Hh signalling pathway?
- disassembly of signalling complexes and involvement of several transmembrane pass receptors.
What is different about Hh synthesis compared to Wnt?
- unlike Wnt Hh is synthesised as a precursor that is subsequently cleaved.
- the thioredoxin side chain of Cys258 performs a nucleophilic attack on the carbonyl carbon of Gly257 to form a thioester.
- the C-terminal portion catalysts linkage of the c-terminus to cholesterol and a palmitoyl group is added to the N-terminus.
How does Hh signalling in drosophila work?
- Hh ultimately activates transcrition of target genes through CI transcription factor in complex with CBP. In the absence of Hh, Prc (the Hh receptor) represses Smo function and Smo is maintained in internal vesicles.
How is CI maintained?
- in a microtubules bound complex with Fu (a kinase) and Cos2 (a kinesiology-like motor protein). CI is phosphorylated in a process involving PKA, GSK3 and CK1.
What happens after CI phosphorylation?
- It is then protolytically degraded by Slimb in a process that requires proteasome.
- the generated Previn fragment, Ci75 represses the transcription of Hh-responsive genes.
What happens after the repression of Hh transcription genes?
- Hh binds to Ptc relieving its repression of Smo. Smo moves to the plasma membrane where it is phosphorylated by PKA and CK1.
- Fu and Cos2 also become phosphorylated and form a complex with Smo.
What happens to this CI-Fu-Cos2 complex?
- it is therefore released from the microtubules.
- CI is not phosphorylated in this complex and the stable protein translocates to the nucleus where it mediates transcrition in complex with CBP.
how does human expression differ from that of the Drosophila?
- express 4 Hh and 2 Ptc as apposed to one of each.
- humans also express 3 Gli transcription factors that appear to perform the function of the single CI of the fly
- Humans do not posses a Cos2 ortholog and the role of Fu ortholog said is uncertain.
- it has been proposed that the primary cilia my (a single long immobile cilium found on most cells) may be a centre for Hh signal transduction.