Ventralisation of the neural tube Flashcards
What is neurulation?
Formation of the neural tube, from the rolling up of the neuroectoderm
What transforms the medio-lateral axis into the dorso-ventral axis?
Neurulation - when the neuroectoderm rolls up to form the neural tube
Where does the notochord lie?
Underneath the neural plate and then comes to lie underneath the ventral midline of the neural tube
What is the ventral floor plate?
A group of specialised glial cells which lie at the midline of the ventral neural tube, above the notochord
How do neurons develop around the neural tube?
With bilateral symmetry, through the entire DV axis
What determines the cell type of cells near the floor plate
- Morphogen gradient from the floorplate and notochord
- Different cell types dependant on what concentration of the morphogen they see
How was is shown that a secreted signal came from the notochord and floor plate and induced cell type?
- Graft a donor notochord into an ectopic position
- Ectopic floor plate and ventral neurons with bilateral symmetry
- Showed that secreted signals from the notochord causes the cells in the neural tube to adopt a different fate
What is the secreted factor from the notochord and floor plate?
Shh
What is special about Shh?
Conserved through evolution
Has homologue in drosophila
Where was Hh discovered and how?
Discovered in drosophila
Mutations in this gene causes the fly to curl up and look spiky (like a hedgehog)
How was it proved that Shh is a secreted molecule? (2 ways)
- Look at the DNA sequence (contains information about what the protein is destined to do)
- Apply antibodies (show protein decorates cells distant to the ones making it in the floorplate/notochord)
What induces Shh expression in the floor plate?
Shh release from the notochord
What does Shh do?
- Diffuses away from the floorplate and notochord
- Setting up a concentration gradient (high concentration ventrally, low concentration dorsally)
- Turns on Shh signalling in neighbouring cells, which up-regulate ventral transcription factors, dependant on the concentration of Shh which they see
- These progenitor cells then ultimately develop into ventral neurons
How was it shown that is was infact Shh coming from the notochord that induced ventral fates and not something else?
- Shh soaked bead produced ectopic floor plate and motor neurons with bilateral symmetry around the midline (along the DV axis)
What do the transcription factors expressed by the progenitors code?
Later differentiation
What do the progenitors give rise to?
1) A daughter cell which remains in the same place as the mother
- Progenitor (neuronal stem cell)
2) A daughter which migrates away laterally to the mantle zone and differentiates into a neuron
What TF are expressed in the daughter cell of a progenitor, which remains at the ventricular zone?
Expresses the same pattern of TFs as the mother
What happens to the daughter cell which moves away from the VZ?
Differentiates into a neuron - change in the TFs expressed
When does ventralisation and dorsallisation of the neural tube occur?
At the same time
How do Shh and BMP act with eachother?
They act antagonistically - repressing each other at the level of transcription factors and at protein level, to set up bilateral stripes of different neuronal classes along the D-V axis, all with different identities
What neuronal fates do BMPs set?
start from closest to roof plate
Dorsal 1 INTERNEURONS
Dorsal 2 INTERNEURONS
Dorsal 3 INTERNEURONS
What neuronal fates does the Shh gradient set?
start from closest to floor plate
Ventral 3 NERUONS
MOTOR NEURONS
Ventral 2 INTERNEURONS
Ventral 1 INTERNEURONS
When does expression of Shh occur?
After the expression of BMPs and neurulation
Where do sensory neurons come into the neural tube?
At the top
How are sensory neurons born?
From migratory neural crest cells which have coalesces to form sensory ganglia (DRG)
Where do relay interneurons lie?
Between the dorsal and motor pools
When are relay interneurons born?
Under the influence of either BMP signalling or low Shh signalling
Where are motor neurons born and how?
Ventrally, in response to high levels of Shh signalling
What happens to neurons once they have acquired their fate and position?
They differentiate further by extending their axons
Where to motor neurons project?
To the periphery
Where do interneurons project?
Within the spinal cord ventrally
What are Ptc and Smo?
Transmembrane receptors
What happens in the cell when Hh signalling is not active?
- Ptc represses Smo
- Intracellular end of Smo cannot interact with downstream machinery
- Majority of Gli TF is in its repressive form
- Gli R represses Hh target genes from being transcribed, by sitting on the promoter of the gene
What is Gli?
What does it do?
A transcription factor which has 2 forms:
- Repressive
- Active
Both forms act non-autonomously to determine the progenitor identify and neuronal fate
What happens in the cell when Hh signalling is active?
- Hh binds to Ptc - preventing the inactivation of Smo
- Intracellular end of Smo shifts Gli in favour of Gli A
- Gli A binds to the promoter of the gene and allows Hh target genes to be transcribed
Why do we want to know the transcription profile of cells that are influenced by Shh?
- If know how much Shh a cell needs to turn a particular set of TFs which define a motor neuron progenitor
- Can take pluripotent cells and expose them to exactly the right amount of Shh, to get a motor neuron
(Beneficial for diseases like motor neuron disease)
Where is Shh expressed in the body?
Along the entire A-P axis
Always ventral in position
Where also expresses Shh in the forebrain?
Special set of cells above the prechordal mesoderm
What happens when Shh is knocked out in the mouse?
- Get 3 germ layers, neural plate, neural tube and dorsally patterning as normal (shh not needed in these steps)
- BUT, get no ventralisation of the neural tube
What are the symptoms in a mouse with no Shh?
1) Holoprosencephaly
- Faliure of the forebrain to form
- No bilateral symmetry of the brain into 2 parts
2) Cyclopia
3) Abnormaly limbs/digits
4) Lack of pituitary
- Abnormal growth and appetite
Why is there lack of pituitary in mice with Shh knockout?
- Midline of the forebrain is the hypothalamus, which sets up the pituitary
- Forebrain doesn’t form
What do patients with holoprocencephaly or cyclopia have an abnormality in
Shh or the Shh pathway
What does the notochord become at the rostral end and what does this induce?
- The prechordal mesoderm (PRECHORDAL PLATE)
- Induces Shh expression in the overlying forebrain cells
Why are there different types of neurons at the same D-V position along the A-P axis, even though Shh governs ventralisation along the entire AP axis and is at the same concentration in the notochord?
- There is an earlier established A-P patterning of the neural tube (Hox code)
- Forms a Cartesion grid of information, along with the patterning information in the D-V axis
(see different Hox genes, along with different concentrations of Shh morphogen)
- Cells respond by changing fate, depending upon there position in the grid and what TFs the cell sees
How is Shh detected better, with antibodies or mRNA?
With antibodies
How do Gli R and Gli A interact?
They both bind to the promoter of the gene
Gli R masks the activation site from Gli A when no Hh is present
What do neurons do as they migrate laterally?
Differentiate into neurons which are ‘preprogrammed’ by the subset of transcription factors which they express
In patterning of the DV axis of the neural tube, what is Shh/BMP/Wnt signalling converted to?
A GliR- GliA gradient
