Which organ is most susceptible to pertubation of laterality?
Heart:
- normal situs solitus
- mirror = situs inversus
- diferent levels of heterotaxy
What breaks the symmetry of the embryo?
establishment of expression of side determinants in the lateral plate mesoderm (Nodal, Lefty, Pitx2)
How is LR organizer called in mice/zebrafish/Xenopus/chick?
mice: node
zebrafish: Kupffer’s vesicle
Xenopus: gastrocoel roof plate
chick: Hensen’s node
What are the general 3 steps of establishing polarity?
- planar cell polarity breaks the symmetry of the node
- flow of Nodal transfers the signal
- establishment of Nodal, Lefty and Ptx2 zones
What are the first molecular signs of lateralization?
Nodal and Cerl2 expression; stronger on opposite sides
Describe Nodal signaling pathway.
- Nodal type II kinase phosphorylates type I receptor.
- Smad2 is phosphorylated, forms complex with Smad4
- Smad4 in nucleus triggers gene transcription
List 4 Nodal target genes.
Cerl2 (inhibits Nodal)
Lefty (inhibits Nodal)
Ptx2
Nodal (positive feedback loop)
What is the role of nodal cilia in lateralization?
Motile monocilia:
- usually 9+0 conformation
- move in rotational manner
- -> establish a leftward-directed fluid flow within the cavity of the L/R organizer
Why is flow established by cilia in the node leftwards?
Because of cilia positioning:
established by PCP pathway that orients the centrosome and the cillium itself (asymmetric divisions)
Describe PCP pathway.
It is a non-canonical Wnt pathway:
- transmembrane cadherin Flamingo can recruit either Frizzled (Wnt receptor) or Van Gogh (also receptor)
- Frizzled can recruit Dsh/Van Gogh can recruit Prickle
- 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
How is the flow of fluid in the node translated into a lateralized Nodal expression?
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
How is Nodal signal relayed to lateral plate mesoderm, where there is no Pkd2?
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
What interactions regarding lateralization happen in the lateral plate mesoderm?
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.
