Cell-cell interactions: TGFbeta and FGF signalling Flashcards
What are the 3 common features in signal transduction pathways?
1) Reception
2) Transduction
3) Response
What happens in the reception stage of a transduction pathway?
Ligand binds to a cell surface receptor and activates it
What can a ligand be?
Extracellular or membrane bound
What happens in the transduction stage of a transduction pathway?
Signal is relayed from the membrane to the nucleus
Via a cascade of secondary messengers
What happens in the response stage of a transduction pathway?
Transcription factor is activated:
- Translocates into the nucleus
- Induces transcription of specific target genes (to activate or repress genes)
What 3 families of molecules fall into the ‘TGF beta superfamily’?
1) BMP-like family
2) GDNF family
3) TGF-beta-like family
What are the members of the BMP-like family of TGF beta molecules?
What are they involved in?
- BMP
- GDF
Involved in development
What are the members of the TGF-beta-like family of TGF beta molecules?
What are they involved in?
- TGF-beta
- Nodal
- Activin
- Veg1
Important role in cancer
What do the members of the TGF beta superfamily share?
What is the exception?
The same mechanism of signal transduction
With the exception of the SMAD proteins that are used - different receptors, different SMAD proteins
Describe the signalling transduction cascade of TGF beta
1) Ligand encounters receptor - first binds to type II receptor
2) Causes the receptor to undergo oligomerisation
- Bings in type I receptor
- Forms dimer with type II receptor
3) Dimer results in the activation of the type II receptor
4) Phosphorylation of type I receptor - triggers overall activation of the receptor
5) Specific SMAD proteins recruited and phosphorylated by the activated receptor
6) SMADs are further activated by SMAD 4 - resulting in an active complex that acts as a transcription factor
7) TF translocates into the nucleus - activating transcription
What activity does the type II receptor have?
What does this promote?
Kinase activity
Promotes the phosphorylation of the type I receptor, triggering the overall activation of the receptor
What is the type II receptor of TGF beta?
Type I receptor?
TGFBR2
TGFBR1
What results in activation of the type II receptor?
Oligomerisation of type I and type II receptors
What are the SMAD proteins that are recruited and phosphorylated by the receptor in the case of TGF-beta ligand?
SMAD 2
SMAD 3
What are the SMAD proteins that are recruited and phosphorylated by the receptor in the case of BMP or GDF ligand?
SMAD 1
SMAD 5
SMAD 8
How is BMP signalling controlled?
Through INHIBITION:
- Regulated extracellularly by antagonists
- Modulate the amount of interaction that BMP and their receptors have
- Different cell fates are dependant on the amount of ANTAGONIST present
What are the antagonists that control BMP signalling?
- Chordin
- Noggin
- Follistatin
- Cerberus
Why is BMP signalling tightly regulated?
How is this easily done?
Want SPECIFIC cells to respond to signal at a SPECIFIC time and a SPECIFIC place
Easily done by having the components of the signalling pathway present but controlling the activity of these proteins through EXTRACELLULAR INHIBITORS
How many families of RTK are there?
What does each family have different?
20 families
Each have individual receptors and ligand partners
What are examples of RTKs?
- Insulin receptor
- NGF receptor
- PDGF receptor
- FGF receptor
- EGP receptor
How do RTKs and their ligands interact?
Some ligands - specific to one receptor, or bind to many
Some receptors - specific to one ligand, or to many
What are 4 common features of all RTKs?
- Mostly monomers (except insulin receptor)
- Extracellular domain varies greatly
- Intracellular domain has kinase activity
- Short, single transmembrane domain
Even though the extracellular domains of RTKs vary greatly, what can be seen?
Recognisable features, like the presence of immunoglobulin domains
How can the kinase domain of an RTK be split?
Into 2 domains
Describe canonical RTK activation
- Ligand dimerises and facilitates dimerisation of receptor
- Once positioned correctly, the kinase domains of the receptors phosphorylate each other (cross-phosphoylation)
How is the dimerisation of RTK receptors different to TGF-beta receptor?
RTK:
- Dimerisation happens simultaneously, through the binding of a ligand
TGF-beta:
- Ligand binds one receptor and recruits the other
What 3 things does ‘cross phosphorylation’ of the RTKs cause?
1) Increases ACTIVITY of the kinase
2) STABILISES the receptor in an active state (ligand independant)
3) Allows the kinase to PHOSPHOYLATE other TYROSINES in the RECEPTOR - creating docking sites
What are the docking sites on the intracellular portion of RTKs for?
Recruitment of secondary messengers
How does the phosphorylated tyrosine receptor transduce the signal?
- Phosphorylated tyrosine acts as a docking site for downstream proteins
How do downstream proteins recognise the docking sites on RTKs?
Contain SH2:
- Recognises the phoshotyrosine
- Recognises glutamic acid - glutamic acid - isoleucine
Name 3 proteins that bind to RTKs
1) PI 3-kinase
2) GTPase - activating protein (GAP)
3) PLC-gamma
What pathway is PI 3-kinase involved in?
Inositol
What pathway is GAP involved in?
RAS/MAP-kinase
What pathway is PLC-gamma involved in?
Inositol
When is ras active?
When it is bound to GTP
What is ras inactive?
When it is bound to GDP
What system is needed for ras activation and why?
A system to propagate the activation of ras in a more STABLE manner
As when GTP is bound by ras it is rapidly hydrolysed - not active for very long
What is Raf?
What is it activated by?
Map- KKK
Activated by GTP ras
What is Mek activated by?
Through phosphorylation by Map-KKK (Raf)
What is MapK activated by?
Dual phosphoylation of SERINE and THREONINE (separated by one amino acids) by Map-KK (Mek)
How many members of FGF ligands are there?
22 members
What are FGFs important in?
Embryonic development
What types of molecules are FGF ligands?
- Small groups of endocrine, juxtacrine FGFs
- Very large group of paracrine FGFs
Describe the expression of some FGFs?
Very DISCRETE
When does FGF8 play an important role?
What does FGF8 play a role in?
During development
- Patterning the midbrain-hindbrain boundary
- Patterning somites
- Patterning the developing limb bud
Where is FGF8 present?
What does it do?
- At the midbrain-hindbrain boundary
- Strong expression in the developing limb bud field
- Specific, segmented pattern of expression in the developing SOMITES
What is the FGF receptor family characterised by?
3 repeat structures in the extracellular domain:
- D1, D2, D3
What are the D1, D2 and D3 structures in the extracellular domain of FGF receptor superfamily?
Immunoglobulin like domains
Where do ligands bind to in the FGF receptor?
D2 and D3 repeat
What kind of receptor is an FGF receptor?
RTK
How many transmembrane domains do FGF receptors have?
1
How many FGF receptors trigger the effect of the 22 FGF ligands?
What does this mean?
4
One FGF receptor binds to MANY FGF ligands
What dictates the specificity of the FGF signalling pathway?
- FGF receptors must form complexes with HSPGs to be able to signal
- HSPGs have specific structures
What do HSPGs modify?
Extracellular cell-cell signalling
What is the structure of HSPGs?
How does this confer specificity to FGF signalling?
- Protein core
- Large sugar chains of heparin attached to the protein core
- Each sugar chain can be modified by sulphation
- Results in a ‘code’ that creates binding sites for specific proteins (eg. FGF2)
SO, have distinct protein core, distinct sugars attached and distinct sulphation modifications
How can the different protein cores of HSPG be arranged?
3 different types:
- Transmembrane
- Tethered
- Secreted
How do the FGF family of receptors transduce the signals?
Through distinct secondary messengers
What does the downstream effect of the FGF receptor depend upon?
Different depending on the secondary signals that are activated
Same ligand/receptor set can trigger COMPLETELY different responses, depending on the secondary messengers activated
What does the Map kinase pathway lead to?
How is this different to other secondary messengers?
Cell proliferation
Other secondary messengers lead to cell survival or cell motility
What are mutations in the extracellular domain of the FGF receptors result in?
Human disease
What does a mutation in FGFR3 result in?
Achondroplasia - congenital birth defect
Why do we need to have an understanding of receptors/ligand functioning and signalling?
Can better understand what does wrong when have mutations effecting particular receptors
