Lecture 10: TGFbeta and FGF signalling Flashcards
Describe the TGFbeta transduction pathway.
TGF-B transduce a signal when they bind at the cell surface
1 – TGFB binds to TGFB receptor 2.
2 – dimerization of TGFB receptor2 to TGFB receptor 1. Close proximity to each other.
3 – receptor 2 phosphorylates receptor 1 on the serine or threonine residues of the cytoplasmic tail
4 – phosphorylation activates the receptor complex
5 – phosphorylation of SMAD2 and SMAD3 in the cytosol
6 – SMAD2 and SMAD3 form a complex with SMAD4 (co-SMAD). SMADs are TFs. If you have a BMP signal binding to BMP receptor 1 and 2, SMAD4 is activated but SMAD1 and SMAD5 are phosphorylated and activated (instead of SMAD2 and 3).
7 – SMAD complex activates TFs which translocate to the nucleus where they bind to specific sequences located upstream of specific target genes. Cause activation and transcription of the target gene.
8 – Downstream = induction of TGF-B responses. Can diverge towards other downstream effectors leading to a slightly different response.
Describe the role of inhibitors in TGFbeta signalling.
In BMP/TGF signalling there are many inhibitors e.g., chordin and noggin. They bind to the ligand and prevent it binding onto its receptor. These inhibitors are present at very restricted places during embryonic development in specific groups of cells. Through their diffusion they control the level of response to BMP signalling that occurs to fine tuning the level of response to BMP signalling important for cell fate choice.
Important for shaping and dorsalising the mesoderm.
What is an FGF? Discuss some FGF receptors.
FGF = fibroblast growth factor.
The FGF receptor family = small family of RTKs that mediate the effect of the large family of FGFs. They have an EC domain of 3 immunoglobulin-like containing domains (D1-3). D1 and D2 separated by acid box which has a role in suppressing the receptor in absence of ligand. D2 and D3 are responsible for binding of ligand. Kinase domain is split into two domains.
Activation of FGF receptors triggers many downstream events, including:
A = activation of Ras -> Raf -> Mek -> Erk pathway
B = recruitment and activation of different mediators.
C = receptor can directly act
Through various downstream pathways, FGF has the power to trigger different processes within the cell.
Discuss the signalling transduction pathway of RTKs.
RTK = receptor tyrosine kinase
Ligands either bind to a single RTK or multiple (insulin). There are many RTKs in the human genome. The EC domains vary greatly – ligand binding. The IC domains are highly conserved and have enzymatic kinase activity.
RTK Activation
Binding of ligand can cause the receptor units to come close to each other. The receptor dimerised causing change in confirmation, activating IC kinase.
Kinase domains of each monomer cross phosphorylate. This is important to increase kinase activity of each receptor and also stabilises it in an active state => active irrespective of the presence of a ligand. Once activated, they cause the phosphorylation of tyrosine residues located in the cytoplasmic domain of the receptor. This creates an SH2 domains = docking sites for IC molecules.
Signal Transduction
The phosphorylated RTK acts as a docking site for IC signalling proteins. E.g., GTPase-activating protein (Ras/Map pathways). It has several domains:
SH2 domain = docking site
SH3 domain = mediates interaction
Explain Ras activation/inactivation.
Ras – two states:
- Active state, bound to GTP
- Inactive state, bound to GDP
Two sets of enzymes essential in controlling the switch state = GEF and GAP
GEF = guanine nucleotide exchange factor – exchanges GDP for GTP.
GAP = GTPase activating protein – returns Ras to inactive state by exchanging GDP for GTP.
Describe the Ras pathway.
- Phosphorylated dimeric RTK contains docking sites for GBR2 which has an SH2 domain
- A GEF (called Sos) binds to GBR2, recruiting Ras
- Sos promotes the dissociation of GDP from Ras, and the binding of GTP, rendering it active
- Sos then dissociates from active Ras
Explain the MAP kinase signalling pathway.
MAP kinase or ERK needs to phosphorylated on two residues, serine and threonine, which are separated by only one AA. This is unlikely to happen by accident => activated by phosphorylation by another kinase called MAP kinase kinase OR MEK in mammalian cells.
MEK is maintained in an inactive form unless phosphorylated by MAP kinase kinase kinase or RAF in mammalian cells.
Raf can only by phosphorylated by binding of Ras. This shows how Ras binding can be translated into long-lived effects.
Activating MAP kinase will activate TF which lead to transcription of target genes; OR activating 2ndary messengers which lead to activation of other TFs.
Explain the importance of HSPGs in FGF signalling.
FGF ligands can only bind and activate FGF receptors if they are associated in a complex with HSPGs (heperan sulfate proteoglycan). Only paracrine FGFs have a high affinity with HSPGs as diffusion can’t occur because they are bound.
HSPGs
- form either TM molecules, a membrane-tethered molecule or a secreted molecule.
- protein core and a long sugar chain. Each sugar chain can be modified in various ways.
Modification of sugar chains provides a code which specifies which ligand binds and causes activation of specific receptors.
