5. L-R asymmetry Flashcards
What is the assymetry in bilateria?
Bilateria are bilaterally symmetrical but have handed asymmetry
Asymmetry in body organs
Define heterotaxy
Heterotaxy - reversal of handedness asymmetry - organs on opposite side than supposed to be
However, in embryos heterotaxy is lethal -> do not develop
What is an example of possible heterotaxy in animals?
Pond snails - their shell + organ organisation can be left / right twisted - dextral or sinistral
Explain LR handedness establishment in Xenopus
In Xenopus LR asymmetry is established:
1) D-V axis established by sperm entry site - dorsal side
2) Further patterning establishes A-P axis
3) LR established relative to D-V and A-P axes by alignment of “F” factor - chiral molecule / structure: align F - drive output of “F” - because of its chirality -> establish LR
Explain how handedness is established in chick
Chick establish handedness in gastrulation - initial cue (?) activates:
- Shh: stage 4 expressed everywhere - stage 5 cells movements move Shh to the future left side - which in turn activates Nodal - Nodal determines left + activates Pitx-2 which sets up organ coiling, looping
- activin: on the right side - inhibits Nodal expression beyond midline into the right side
In chick Nodal establishes left side + patterns mesoderm there
No conservation of signalling upstream of LR establishment - only conserved functions of Nodal and Pitx2
What is the usual role of Nodal in development? What would be an example of unusual use of Nodal
Nodal usually specifies mesoderm
BUT in chick establishes **L handedness **
+ specifies mesoderm on the left later
Explain how is LR handedness established in mice
In mice L-R handedness established
IV gene encodes L-R dynein protein -> in mouse gastrulation - in the Node cavity dynein arranged in monocilia - monocilia at an angle + directional spinning - slanted monocilia generate clockwise beating which creates R->L flow of liquid -> mechanical pressure bends the border of left monocilia - Nodal expression on left side
Mouse IV mutants have random handedness establishment
Explain how do monocilia generate flow in mice LR establishment
IV gene codes for dynein - dynein arranged in monocilia with chiral microtubules - monocilia arranged at an angle + spin clockwise - generate liquid directional flow in the node cavity
Define what is planar cell polarity
Planar cell polarity (PCP) - coordinated orientation of cells within the same plane of a tissue - as opposed to apical-basal polarity, which is the orientation of cells along the vertical axis (top to bottom)
How is trichome polarity established in Drosophila wing?
Trichome polarity in Drosophila wing established by planar cell polarity (PCP) - a classic example - cells coordinate their extension orientation in a tissue plane - use Wnt/Frizzled signalling pathway:
proteins arranged in cell membranes - Van Gogh (Vang) and Frizzled (Fz) on opposite membranes of the cell - repulse each other in the same cell - maintain each other in neighbouring cells
Fz and Vang - core mechanism in PCP
Explain how Fz and Vang act in PCP in mice
In mice PCP established in the node - cells need to locate their monocilia to the posterior side of the cell - same mechanism as in Drosophila wings - Fz and Vang repulse each other in the same cell but mantain in neighbouring cells
Vang- mutant -> monocilia in cell centre instead of posterior - irregular flow -> Nodal displacement -> handedness defects
What animals use directional liquid flow for establishing LR asymmetry in development?
Mice, Xenopus
Explain the experiment done to study Xenopus PCP knockdown
Disrupt PCP - Vang-/- disruption - by injecting Xenopus 4-cell embryo each cell with RNA (Xenopus can’t be genetically modified ??) - also Myosin1d-/- -> monocilia short and central in the cell instead of P -> flow disrupted - Nodal displaced => handednesss defects - LR reversal Heterotaxy
When does Xenopus establish LR asymmetry?
LR asymmetry in Xenopus:
1) maternally deposited RNA for ATP4-alpha ion pump - microfilament and microtubule arrays arrange ion pumps in the embryo - microtubules chiral themselves - not only +/- poles but also chiral - can push molecules to specific sites -> ion pumps localised by cytoskeleton create electrical gradient - seratonin localised in right ventral
-> however, this result has not been replicated - could be wrong
2) monocilia induced directional fluid flow and Nodal
Which species is different in handedness establishment compared to others?
Chick, reptiles - no monocilia, no liquid flow in Hensen’s node
Less understood mechanism than monocilia
How is cell movement involved in establishing handedness in chick?
At embryo stage 4 Shh is expressed all over the embryo - stage 5 Shh localise more to the future left - this change associated with cell handed movement from right to left - Shh activates Nodal expression on the left - specifies the left
Asymmetry established before Nodal expression - Shh already different at stage 5
Disrupt Shh / cell movement / Nodal -> dirupt LR asymmetry
Explain how LR asymmetry is established in Drosophila
No Nodal gene in Drosophila - lost in evolution -> not the monocilia mechanism
**Less handed asymmetry **than in other animals but still gut coiling + male genitalia rotation asymmetrical: Myosin 1d molecular motor in F-Actin and Vang in PCP determine Droosphila handedness
Explain how LR asymmetry is established in pond snails
Pond snails - one of few species which are polymorphic in their handedness - LR asymmetry in shell + internal organ organisation - sinsitral / dextral - have different mechanisms for each
Sinistral establishment: dd genotype -
at 4-8 cell stage no twist - later development of the sinistral twist
Dextral extablishment: Dd / DD genotype - at 4-8 stage spiral cleavage - twist in cell divisions
=> cell divisions establish LR asymmetry in snails
Explain what was Davison paper looking into in Snail handedness establishment
Davison et al. were trying to determine the gene responsible for handedness determination - DD/Dd develop dextrally, dd - sinistrally -> mapped genes associated with the phenotype - identified Ldia1 and Ldia2 genes - produce formin - d allele of Ldia2 has a deletion framehsift - shifts all other ORFs -> truncated RNA transcript => sinistral development
Looked into disrupting formin by drug treatment - treated Dd/DD dextral embryos to disrupt formin function - see if developed sinistrally => 25% developed sinistral - vague evidence
Interesting choice of drug treatment rather than genetically engineering - another paper chose genetic engineering - got better results => formin influences handedness development - in sinistral development formin not fully functional - but not the core determinant of LR asymmetry
What is the spontaneous arrangement of mammalian cytoskeleton? What experiment was done
Mammalian fibroblasts cultures - observed how organise their cytoskeleton -> dextral orientation of cytoskeleton - chiral - microfilaments get arranged in a spiral
What is the evolution fo LR handedness mechanisms?
Common ancestor was able to establish LR asymmetry - local coordination specifies R - overall event to differ LR
