Lecture 18 - The TGF-β family in development Flashcards
What proteins are included in the TGF-β family and what are ther functions.
Includes
* Transforming growth factor-β
* Bone morphogenetic proteins (BMPs)
* Activin
* Nodal
* Decapentaplegic (Dpp)
Functions
* Stimulation of cell division
* Inhibition of cell division
* Alter synthesis of growth factors, cell adhesion molecules and extracellular matrix
Induction and cell specification in development (esp. axis formation)
Describe how TGF-B protiens are processed?
TGF-β Family Processing
* Secreted as inactive precursors
* Leader region required for secretion into the ER
* Variable length pro-domain – cleaved in the ER/Golgi secretary pathway
* Mature domain contains 110-140 amino acids
* Must dimerize to be active, either as a homodimer or heterodimer - diversity
Structure includes a cystine knot held together by intra-chain disulphide bonds. The cystine knot stabilises the structure by holding it tight and keeping it compact, it protects the protein from extracellular degradation. It can also be found in other secreted growth factors.
Describe the TGF-B signalling pathway and the associated receptors.
TGF-β receptors
Can be identified by adding radioactive TGFβ to cells
* Type I - 55kD - Transmembrane, serine/threonine kinases
* Type II - 85kD
* Type III - 285kD - Proteoglycan
TGF-β signalling
1. (TGF-β dimer binds to type III receptor)
2. TGF-β dimer binds to dimer of type II receptors
3. Receptor tetramer formed (2 type II and 2 type I)
4. Type II receptor phosphorylates cytoplasmic domain of type I receptor
5. Kinase activity of type I activated
6. R(receptor-regulated)-Smad phosphorylated by type I receptor
7. Phosphorylated R-Smad binds to Co-Smad
8. R-Smad/Co-Smad dimer enters nucleus
9. Smad dimer forms complex with DNA-binding protein
10. Complex binds near target genes and activates transcription
TGF-β Specificity
* Various type I/type II receptors (and heterodimers/homodimers)
* Various R-Smads
* Various co-Smads
* R-Smad/co-Smad combination determines promoter target
○ e.g. TGF-β causes phosphorylation of Smad2 which dimerizes with Smad4
○ e.g. BMP2 causes phosphorylation of Smad1 which dimerizes with Smad4
* Specificity for a given R-Smad is determined by 3 amino acids in the type I receptor
TGF-β family members are involved in axis specification (A-P and D-V) in vertebrates and invertebrates
Describe the process of fertilisation (in amphibian development)
The site of sperm entry determines the dorso-ventral axis in amphibian development
Sperm binding induces microtubule based rearrangements of the cytoplasm
Cytoplasm rotates towards point of entry
Dorsal side develops a signaling centre called the Niewkoop centre
Cleavage divisions then occur
Develops into a blastula
Describe the process of xenopus gastrulation.
Xenopus gastrulation
Forms the gut and the primary germ layers
Invagination (blastopore) to form primitive gut called the archenteron anus ->mouth = A-P axis. So D-V specified by sperm entry point -> subsequent rearrangement of cytoplasm in zygote determines the position of the blastopore
And A-P specified by gastrulation from blastula through gastrula stages
Mesoderm forms from marginal zone – mostly overlying archenteron
Describe the process of neuralation.
Neurulation
* 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 the 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 cord
* Eventually differentiate into dermis, skeletal muscle, cartilage, tendons, and vertebrae……
Somites - bilaterally paired blocks of paraxial mesoderm that form along the head-to-tail axis of the developing embryo in segmented animals
Describe the movement of neural crest cells
Neural crest cells:
* Specialist migratory populations
* Migrate on cranial, dorsolateral and ventral pathways
* Highly migratory, invasive and proliferative
Include melanocytes (called melanoblast in embryo) and cells that form the adrenal medulla
Describe mesoderm formation.
Xenopus
Signal from Vegetal cells to induce Mesoderm formation from cap cells.
There are two different signals from the vegetal cells, dorsal vegetal (Notochord) and ventral vegetal (mesenchyme, epidermis)
The dorsal vegetal region (Niewkoop centre) induces an organiser
Describe amphibian axis formation.
- Sperm entry causes cortical rotation which specifies ventral and dorsal quadrants of vegetal hemisphere
- Nieuwkoop centre forms in dorsal vegetal quadrant
- During blastula stage Nodal-related signalling induces BMP4 in marginal zone animal cap cells
- High levels of Nodal-related induce organiser in dorsal marginal zone
- Organiser secretes Chordin which allows dorsal mesoderm to form
- Xolloid degrades Chordin in ventral cells which allows ventral mesoderm to form
- Organiser forms blastopore and initiates gastrulation
Mesoderm differentiates from marginal zone cells
Describe gastrulation in drosophilia
- Cell fates specified on the DV axis in blastoderm (similar to blastula)
- Gastrulation happens in same way
- Mesoderm folds inwards along ventral midline most ventral part of embryo forms mesoderm
- More ventrally cells can form epidermis or nerves then epidermis only
- Amnioserosa is an extraembryonic membrane
DV axis defined by changes in cell fate (similar to mesoderm in amphibians)
what is the role of Dpp in axis formation
Dpp (BMP4-like) specifies dorsal cell fates
Cell fates specified by genes identified by identifying mutants with DV axis problems
Decapentaplegic (Dpp) is most closely related to BMP4 (homolog)
Whereas BMP4 specifies ventral fate Dpp specifies dorsal fate (Dorsal fates don’t form in mutants)
How is the D-V axis specification conserved?
Conservation of D-V axis specification
Short gastrulation (Sog) is a homolog of chordin
Antagonises Dpp and prevent it from dorsalising ventral tissue
Sog is cleaved by Tolloid protease (Xolloid homolog) to allow dorsalising activity of Dpp
Same proteins despite differing body plans………………
