Ventralisation of the neural tube

Question 1

What is neurulation?

Answer 1

Formation of the neural tube, from the rolling up of the neuroectoderm

Question 2

What transforms the medio-lateral axis into the dorso-ventral axis?

Answer 2

Neurulation - when the neuroectoderm rolls up to form the neural tube

Question 3

Where does the notochord lie?

Answer 3

Underneath the neural plate and then comes to lie underneath the ventral midline of the neural tube

Question 4

What is the ventral floor plate?

Answer 4

A group of specialised glial cells which lie at the midline of the ventral neural tube, above the notochord

Question 5

How do neurons develop around the neural tube?

Answer 5

With bilateral symmetry, through the entire DV axis

Question 6

What determines the cell type of cells near the floor plate

Answer 6

• Morphogen gradient from the floorplate and notochord

- Different cell types dependant on what concentration of the morphogen they see

Question 7

How was is shown that a secreted signal came from the notochord and floor plate and induced cell type?

Answer 7

• 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

Question 8

What is the secreted factor from the notochord and floor plate?

Answer 8

Shh

Question 9

What is special about Shh?

Answer 9

Conserved through evolution

Has homologue in drosophila

Question 10

Where was Hh discovered and how?

Answer 10

Discovered in drosophila

Mutations in this gene causes the fly to curl up and look spiky (like a hedgehog)

Question 11

How was it proved that Shh is a secreted molecule? (2 ways)

Answer 11

• 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)

Question 12

What induces Shh expression in the floor plate?

Answer 12

Shh release from the notochord

Question 13

What does Shh do?

Answer 13

• 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

Question 14

How was it shown that is was infact Shh coming from the notochord that induced ventral fates and not something else?

Answer 14

• Shh soaked bead produced ectopic floor plate and motor neurons with bilateral symmetry around the midline (along the DV axis)

Question 15

What do the transcription factors expressed by the progenitors code?

Answer 15

Later differentiation

Question 16

What do the progenitors give rise to?

Answer 16

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

Question 17

What TF are expressed in the daughter cell of a progenitor, which remains at the ventricular zone?

Answer 17

Expresses the same pattern of TFs as the mother

Question 18

What happens to the daughter cell which moves away from the VZ?

Answer 18

Differentiates into a neuron - change in the TFs expressed

Question 19

When does ventralisation and dorsallisation of the neural tube occur?

Answer 19

At the same time

Question 20

How do Shh and BMP act with eachother?

Answer 20

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

Question 21

What neuronal fates do BMPs set?

start from closest to roof plate

Answer 21

Dorsal 1 INTERNEURONS
Dorsal 2 INTERNEURONS
Dorsal 3 INTERNEURONS

Question 22

What neuronal fates does the Shh gradient set?

start from closest to floor plate

Answer 22

Ventral 3 NERUONS
MOTOR NEURONS
Ventral 2 INTERNEURONS
Ventral 1 INTERNEURONS

Question 23

When does expression of Shh occur?

Answer 23

After the expression of BMPs and neurulation

Question 24

Where do sensory neurons come into the neural tube?

Answer 24

At the top

Question 25

How are sensory neurons born?

Answer 25

From migratory neural crest cells which have coalesces to form sensory ganglia (DRG)

Question 26

Where do relay interneurons lie?

Answer 26

Between the dorsal and motor pools

Question 27

When are relay interneurons born?

Answer 27

Under the influence of either BMP signalling or low Shh signalling

Question 28

Where are motor neurons born and how?

Answer 28

Ventrally, in response to high levels of Shh signalling

Question 29

What happens to neurons once they have acquired their fate and position?

Answer 29

They differentiate further by extending their axons

Question 30

Where to motor neurons project?

Answer 30

To the periphery

Question 31

Where do interneurons project?

Answer 31

Within the spinal cord ventrally

Question 32

What are Ptc and Smo?

Answer 32

Transmembrane receptors

Question 33

What happens in the cell when Hh signalling is not active?

Answer 33

- 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

Question 34

What is Gli? What does it do?

Answer 34

A transcription factor which has 2 forms: - Repressive - Active Both forms act non-autonomously to determine the progenitor identify and neuronal fate

Question 35

What happens in the cell when Hh signalling is active?

Answer 35

- 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

Question 36

Why do we want to know the transcription profile of cells that are influenced by Shh?

Answer 36

- 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)

Question 37

Where is Shh expressed in the body?

Answer 37

Along the entire A-P axis | Always ventral in position

Question 38

Where also expresses Shh in the forebrain?

Answer 38

Special set of cells above the prechordal mesoderm

Question 39

What happens when Shh is knocked out in the mouse?

Answer 39

- 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

Question 40

What are the symptoms in a mouse with no Shh?

Answer 40

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

Question 41

Why is there lack of pituitary in mice with Shh knockout?

Answer 41

- Midline of the forebrain is the hypothalamus, which sets up the pituitary - Forebrain doesn't form

Question 42

What do patients with holoprocencephaly or cyclopia have an abnormality in

Answer 42

Shh or the Shh pathway

Question 43

What does the notochord become at the rostral end and what does this induce?

Answer 43

- The prechordal mesoderm (PRECHORDAL PLATE) | - Induces Shh expression in the overlying forebrain cells

Question 44

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?

Answer 44

- 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

Question 45

How is Shh detected better, with antibodies or mRNA?

Answer 45

With antibodies

Question 46

How do Gli R and Gli A interact?

Answer 46

They both bind to the promoter of the gene | Gli R masks the activation site from Gli A when no Hh is present

Question 47

What do neurons do as they migrate laterally?

Answer 47

Differentiate into neurons which are 'preprogrammed' by the subset of transcription factors which they express

Question 48

In patterning of the DV axis of the neural tube, what is Shh/BMP/Wnt signalling converted to?

Answer 48

A GliR- GliA gradient