Lecture 4-5

Question 1

Which neuronal types differentiate at lower concentrations of Shh. (Posterior spinal cord)

Answer 1

Intermediate neuronal types (V0, V1, V2)

Question 2

Which neuronal types differentiate at higher Shh levels. (posterior spinal cord)

Answer 2

Ventral neuronal types (MN, V3)

Question 3

Outline the events leading up to ligand dependent antagonism

Answer 3

In the absence of Shh, Ptch1 represses Smo activity. After Shh binds to Ptch1 and co-repressors Gas1/Cdon, Smo repression is relieved. GliA therefore isn’t transformed into GliR. GliA modulates transcriptional targets that negatively feedback and downregulate Cdon/Gas1. Receptors Ptch1,2 and HHIP1 are upregulated, these bind Shh ligand. This competes with productive ligand-receptor complexes to alter the balance of bound/unbound Ptch1. This results in cell autonomous modulation of Smo activity. Additionally, ligand is sequestered by Shh antagonists leads to non cell autonomous Shh inhibition in cells distal to the Shh source. (ligand dependent antagonism)

Question 4

What is the non-autonomous effect of ligand dependent antagonism

Answer 4

The increase in receptors sequesters Shh and limits the spread of Shh itself. This results in a shortening and steepening of the Shh morphogen curve. Effectively producing a smaller and more local effect.

Question 5

What is the result of LDA on a set of cells over time

Answer 5

If a cell stops expressing Shh and Shh receptors as a result of LDA then LDA is stopped in that region. This supports the continual spread of Shh into neighbouring territories. Because Shh induces its own expression, this will support a spread of Shh expression to the neighbouring territory.

Question 6

Which enhancer regulates the expression of Shh in the hypothalamus

Answer 6

SBE2

Question 7

What is special about the SBE2 enhancer

Answer 7

It contains a binding site for a T-box transcription factor, Tbx2. This represses Shh by displacing Sox2. GliA and Sox2 both needed to activate Shh transcription, therefore Shh transcription ceases.

Question 8

What induces GliA

Answer 8

Shh and nodal from the early PM

Question 9

What induces Tbx2

Answer 9

BMP7 from the late PM

Question 10

What is the current model for hypothalamic differentiating cells

Answer 10

Fgf10+ progenitor cells downregulate Shh (Tbx2 is expressed in the same cell as FGF10. Cells anterior to this upregulate Shh through LDA, becoming an anterior progenitor, and continually until becoming an anterior cell.