Lecture 2 - Regulating the regulators Flashcards
How does the timing and localisation of MyoD and myf5 expression vary among species? What does this indicate?
Avians
MyoD: expressed first in the medial part of the somite adjacent to the neural tube
Followed quickly by myf-5 and then myogenin
Mammals
myf5 expressed first in somites at 8dpc
myoD expression follows later at 10dpc
Frogs
myoD and myf5 are activated in the early mesoderm
Mice
all 4 MRF genes have different expression patterns during early development
Distinct expression patterns of the MRFs indicate that each has a distinct role during skeletal muscle development
Give an example of the different expression patterns of the MRF genes in mice
myf5 is initially expressed in the dorsal medial (epaxial) mytome very early
myoD expressed ventral lateral (hypaxial) myotome later
Why do myf5 (-/-) mice have delayed skeletal muscle formation?
mrf5 is expressed before myoD in mice
What do MRFs bind?
MRF are BHLH proteins that bind E-boxes (simple base pair)
How do somites form?
Condense from the segmental plate mesoderm as balls of columnar epithelial cells
Happens in the anterior to posterior (rostral to caudal) direction
What do the hypaxial cells of the myotome form?
migrate out and form muscles of the limbs
What is the hypaxial myotome otherwise known as?
the ventral lateral myotome
What is the epaxial myotome otherwise known as?
the dorsal medial myotome
How have lineage studies demonstrated the fate of of the medial and lateral halfs of the somites?
Using chick/quail chimeras (quail cells have dense purple nuclei - replaced medial part of the somite with quail somite) of half somite grafts
- the medial part of the early somite gives rise to the myotome and epaxial muscles
- lateral part of the early somite gives rise to cells that migrate to the limb and for hypaxial muscles
When the medial and laterial parts of the early somite are swapped, the somite reorganises and gives rise to normal myotome and limb musculature. Showing the somite cells are still plastic in the early somite and the local environment influences their speciation
How did Powell demonstrate visually that there may be an external signal influencing the speciation of the somites?
- In situ hybridsation
- for MyoD in newly segmented somites: expression is adjacent to the axial structures ) NT/NC
- Embryological manipulations
- If move the somite, respecifies because of the signals around it
What is the environment of the somite?
Medial part of the somite up against the dorsal midline (axial) structures: neural cord tube and notochord
How do somites form?
Condense from the segmental plate mesoderm in balls of columnar epithelial cells
In an anterior to posterior direction (rostral caudal)
What is an inducer and what is the mesoderm induced by?
Mesoderm: FGF, nodals
Inducing: cell signalling tells cells to be a certain part e.g. mesoderm/ectoderm
What did Powell show about MRF expression in the early somite?
Both Myf5 and MyoD on in the early somite
MyoD only expressed in the medial domain
If move the early somite it respecifies because f the signals around it
What did Powell demonstrate about the MRF expression in the somites by embryological manipulations?
Used embryological manipulations to demonstrate that the mideline structures (specifically the notochord) are required for the activation and maintenance of MRF gene expression in the somites
What was the process of embryological manipulations that Powel used to show that the midline structures (specically the notochord) are required for expression and maintenance of MRF gene expression in the somites?
Embryological manipulations
- Manipulated the embryo so that somites on one side of the midline structures (neural tube and notochord) were separated from the midline structures - so that no signal coming from the midline structures could reach the somites
- In the somites that were left next to the midline structures, expression of MyoD is maintained (ISH) but in the case where the somite developed away from the notochord MyoD and Myf5 not activated
PART 2: The notochord is required for the activation of mrf gene expression in the somites
- Split neural tube so that half somites developed with adjacent to the neural tube alone, and half next to the neural tube and notochord; also removed the notochord: found that only somites that developed adjacent to the notochord showed expression of myoD (although there was somitagenesis without the notochord)
If notochord is removed lose expression of myoD
Where is sonic hedgehog produced?
Shh is produced in the floorplate and the notochord
How is the extracellular Shh percieved by the responding cells and how was this elucidates?
Mutagenesis screens in drosophila
Identified a number of genes in the hedgehog pathway
- patched (ptc), smoothened (smo) and cubitus interuptis (ci)
- without hh, ptc repressed smo and the cos/fu microtubule complex causes the transcription factor Ci to be processed into the smaller form which repressed hh target genes
- in the presence of hh binding to ptc (transmembrane receptor), ptc is inhibited and smo is activated. Smo interacts with cos/fu complex (group of proteins associated with microtubules to disrupt the assocation with the microtubules), resulting in the stabliisation of the full length Ci which can move to the nucleus and function as a transcriptional activatior of hh target genes
How is the hh signalling pathway further maintained when it is active by hh binding to ptc?
ptc is one of the target genes for hh signalling, maintaining hh pathway
How do we investigate whether a gene is important for a particular process?
If in mice, do a knockout of the gene
What was the purpose of the Borycki study?
Knock out shh in mice and evaluate whether myf5 or myoD expressed is affected in the somites
What did Borycki demonstrate about shh null mice?
Neither myf5 nor MyoD are expressed in the epaxial dermamyotome
but are expressed in the hypaxial dermamyotome
Conclusion: Shh needed for the expression of myf5 and MyoD in the epaxial dermatome
What critism was there of Borycki’s conclusion that shh is needed for epaxial expression of myf5 and myoD ? How was this critism disproved?
The cells may just be dying as shh is (or was thought) to be necessary for survival
Assay for cell proliferation - BrdU labelling
- Put uracil into embryos with a modification so that it can be observed. Inject pregnant mothers with BrdU which gets taken up if dividing at the time of injection (both WT and shh (-/-)
- Mitotically active cells found in the dermamyotome of both WT and shh null mice
Assay for lineage specific apoptosis in the epaxial dermamyotome in shh null mice
- Used an antibody based assay (TUNEL staining). When cells die, DNA begins to form dsbreaks. TUNEL staining chemically labels the end of broken DNA for the detection of apoptosis in tissue sections
- Apoptotic cells found in the ventral somite in both WT and shh null mice, not in the dorsal somite
- Therefore apoptosis does not explain the failure of the epaxial cells to activate myf5 and myoD expression
What does TUNEL staining stand for?
Terminal deoxynucleotidyl transferase-mediated dUTP Nick End-Labelling