L18: The TGF-beta family in dev Flashcards
1
Q
TGF-beta family: key members and general functions
A
- Bone morphogenetic protein (BMPs)
- Activin
- Nodal
- Decapentaplegic (Dpp)
… - Stimulation of cell division
- Inhibition of cell division
- Alter synthesis of GFs, cell adhesion molecules and E-C matrix
- Induction and specification in development (esp. axis formation)
2
Q
Processing of TGF-b
A
- Secreted as inactive precursors, w/ leader, pro-domain and mature domain
- Leader region required for secretion into the ER
- Variable length pro-domain (cleaved in the ER/Golgi secretory pathway)
- Must dimerize to be active, either homo- or heterodimer (disulphide bond)
3
Q
TGF-b signalling pathway
A
- Dimers binds Type II rec.
- Allows interaction w/ type 1 rec.; tetramer forms w/ type 1 rec.; type 1 phosph. activating its kinase activity
- Phosph. R-Smad
- r-Smad recruits co-Smad, DNA binding partner
-> binds near target gene, transcription occurs
*type 3 rec. can be present, affects availability of TGF ligand for its rec.
4
Q
TGF-b specificity
A
- Various combinations of typeI/II rec.s, R-smads and co-smads
- e.g TGF-b causes phosph. of Smad2 which dimerises w/ Smad4
- e.g. BMP2 causes phosph. of Smad1 which dimerises w/ Smad4
- Specificity determined by 3 AAs in type I rec.
5
Q
Steps in fertilisation process
A
- Point of sperm entry defines ventral aspect
- Sperm binding induces m.tub based rearrangements of cytoplasm
- Cytoplasm rotates towards point of entry
- Dorsal side develops a signalling centre (‘Niewkoop centre’)
- Cleavage divisions then occur
-> blastula develops
6
Q
Xenopus gastrulation
A
- Begins from invagination (‘blastopore’), forming primitive gut called archenteron
- Begins with anus, ends with mouth (A-P axis specified by gastrulation from blastula)
- D-V axis specified by entry of sperm (subsequent rearrangement of cytoplasm in zygote)
- Mesoderm forms marginal zone (mostly overlaying archenteron)
7
Q
Neurulation
A
- Most dorsal region of the embryo becomes the neural plate
- Neural folds develop and eventually fuse to form the neural tube
- Brain forms at anterior and rest forms the spinal chord
- The neural plate bends up and later fuses to form the hollow tube that will eventually differentiate into the brain and the spinal chord
8
Q
Somites
A
- Bilaterally packed blocks of paraxial mesoderm that form along the head-to-tail axis of developing embryo in segmented animals
- Differentiate into dermis, skeletal muscle, cartilage, tendons and vertebrae
- Gradual process, starting from head
9
Q
Neural crest cells (What are they? What are their key characteristics? Name the 4 types of NC cells)
A
- Form from most dorsal aspect of neural tube (mid-gestation)
- Specialist migratory populations
- migrate on cranial, dorsolateral and ventral pathways
- Highly migratory, invasive and proliferative
- Types: Cranial, vagal, trunk, sacral
e.g. melanocytes and cells that forms adrenal medulla
10
Q
Fate decisions in mesoderm (evidence)
A
- Marginal zone isolated into culture
- Explants from vegetal cells shown to induce mesoderm formation from animal cap cells
- (early blastula alone only begets ectoderm and vegetal tissue)
- Evidence shows that there are two different vegetal cell signals (dorsal vegetal and ventral vegetal)
-> experiments w/ the cells from the two regions generate different populations of cells (according to V<->D)
11
Q
The organiser (How does it develop? Evidence for its roles)
A
- Induced by Niewkoop centre in the dorsal vegetal region (opposite sperm binding)
- Occurs after 30 degree cortical rotation
- Blastopore forms w/ Spemann organiser at dorsal-most point (purported to be involved in specification of new axis; transplantation of organiser induces a new axis -> two heads)
- Alters fate of ventral mesoderm (explant of dorsal marginal zone can induce dorsal fate in ventral region i.e. coculture grew lots of muscle)
-> dorsalising signal released
12
Q
Mesoderm inducing signals
A
- Ventral
- Dorsalising (Organiser)
- Dorsal (Nieuwkoop)
-> TGF-b and their antagonists
13
Q
Examples of TGF-b in mesoderm specification (x4)
A
- Nodal related, primary signal from vegetal hemisphere (induces mesoderm in marginal zone)
-> HIGH: dorsal mesoderm/organiser
-> LOW: ventral mesoderm - BMP4, induces ventral mesoderm formation
- Chordin, antagonises BMP4 and dorsalises mesoderm
- Xolloid, metalloprotease that cleaves chordin allowing BMP4 to act
14
Q
Process of mesoderm induction
A
- BMP4 expr. in marginal zone, inducing ventral mesoderm
- TGF-b members induced before fertilisation induce nodal-related in Veg hemisphere
- b-Catenin boosts nodal-related in NK centre
-> DV gradient, induces dorsal mesoderm and Organiser - Nodal-related induces Organiser and dorsal mesoderm
- Organiser produces Chordin which breaks down BMP4, allowing dorsalisation of region
- Xolloid cleaves Chordin allowing BMP4
15
Q
How do Chordin and Xolloid regulate BMP4?
A
- Chordin binds to BMP4, preventing it from binding to its type I and II rec.s
- Xolloid causes proteolysis of Chordin, restores function of BMP4 as it is no longer bound
