Lateralization

Question 1

Which organ is most susceptible to pertubation of laterality?

Answer 1

Heart:

• normal situs solitus
• mirror = situs inversus
• diferent levels of heterotaxy

Question 2

What breaks the symmetry of the embryo?

Answer 2

establishment of expression of side determinants in the lateral plate mesoderm (Nodal, Lefty, Pitx2)

Question 3

How is LR organizer called in mice/zebrafish/Xenopus/chick?

Answer 3

mice: node
zebrafish: Kupffer’s vesicle
Xenopus: gastrocoel roof plate
chick: Hensen’s node

Question 4

What are the general 3 steps of establishing polarity?

Answer 4

1. planar cell polarity breaks the symmetry of the node
2. flow of Nodal transfers the signal
3. establishment of Nodal, Lefty and Ptx2 zones

Question 5

What are the first molecular signs of lateralization?

Answer 5

Nodal and Cerl2 expression; stronger on opposite sides

Question 6

Describe Nodal signaling pathway.

Answer 6

• Nodal type II kinase phosphorylates type I receptor.
• Smad2 is phosphorylated, forms complex with Smad4
• Smad4 in nucleus triggers gene transcription

Question 7

List 4 Nodal target genes.

Answer 7

Cerl2 (inhibits Nodal)
Lefty (inhibits Nodal)
Ptx2
Nodal (positive feedback loop)

Question 8

What is the role of nodal cilia in lateralization?

Answer 8

Motile monocilia:

• usually 9+0 conformation
• move in rotational manner
• -> establish a leftward-directed fluid flow within the cavity of the L/R organizer

Question 9

Why is flow established by cilia in the node leftwards?

Answer 9

Because of cilia positioning:

established by PCP pathway that orients the centrosome and the cillium itself (asymmetric divisions)

Question 10

Describe PCP pathway.

Answer 10

It is a non-canonical Wnt pathway:

1. transmembrane cadherin Flamingo can recruit either Frizzled (Wnt receptor) or Van Gogh (also receptor)
2. Frizzled can recruit Dsh/Van Gogh can recruit Prickle
3. Dsh competes with Prickle and Diego (protein in cytoplasm):
   - Diego promotes Frizzled signaling
   - Prickle inhibits Frizzled signaling

In the node:
Prickle found anteriorly
Diego found posteriorly

= estabishes positioning of the cilia

Question 11

How is the flow of fluid in the node translated into a lateralized Nodal expression?

Answer 11

need Cerl2 (Nodal antagonist) and Pdk2 (Ca channel)

Pkd2 = Ca channel
required for sensing the nodal flow:
- if cilia tilted, Ca influx inhibits Cerl2 = more Nodal on the LEFT

Question 12

How is Nodal signal relayed to lateral plate mesoderm, where there is no Pkd2?

Answer 12

2 possible routes:
external (wrong)
internal (correct)
- Nodal secreted to the basolateral side
- transmitted to the LPM via paraxial mesoderm
– paraxial mesoderm is unresponsive to Nodal

Question 13

What interactions regarding lateralization happen in the lateral plate mesoderm?

Answer 13

self-enhancement lateral inhibition type of reaction-diffusion system

Nodal and Lefty both expressed on the left side, 
both TGF-beta related proteins
but opposite functions:
- Nodal = left side determinant
- Lefty = Nodal antagonist
--> compete for interaction with cryptic
--> lead to signaling through FoxH1
--> lead to transcription of nodal (positive feed forward loop) and lefty (negative feedback  loop)

Generate gradient.
Maintained via Pitx2.