5-350 Lecture Notes - Activin And Inhibin, Kinesin, Bone Morphogenetic Protein 4

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Outline of Lecture 09
Spatial Patterning in Vertebrates
I. A/P Patterning (role of Hox)
A) Previously primitive node was thought responsible, but evidence points to
AVE; Hox genes are also important in establishing the A/P pattern
B) Hox gene structure: see lecture 07, esp. collinearity with expression and RA
sensitivity
C) Hox Code
- Theory that the combination of expressed Hox genes determines dev. pathway
for that region (fewer HOX genes expressed anteriorly, more expressed
posteriorly)
- Evidence: loss of function gives more anterior structure; gain of expr by retinoic
acid gives more posterior structure; boundaries of Hox expression and
segmental boundaries coincide
D) Regulation of Hox genes: as node regresses posteriorly, RA levels increase,
and this activates the more posterior Hox genes
II. D/V Patterning
A) Role of Shh in neural tube development
- Sonic hedgehog (Shh) is expressed in notochord, then in neural floor plate
(ventral part of neural tube)
- Both of these tissues can induce neural floor plate formation
- Concentration of Shh determines fate of cells (ventral tissues have highest Shh
exposure)
B) Inductive signals in D/V patterning of neural tube
- BMP4 and BMP7 (TGF-β family) are candidate dorsal inducers
- BMPs activate expression of DSL1, also a dorsally expressed gene
- On the ventral side, Shh acts as the initial inducer
C) Somite patterning by Wnt
- Both Wnt and Shh are needed for myotome (middle component of
somite) development
- Shh induces pax-1 expression in sclerotome
III. Left-Right Patterning (not well understood)
- Left-Right inversion mutants provide some clues to how patterning occurs
- Some genes are expressed LR asymmetrically, e.g. nodal, lefty-2, pitx2, Sn-R
- Flow of nodal from R to L in the node may be important to patterning; KIF3B (a
kinesin) is needed for cilia that mediate the flow
- Shh which is expressed on the left is repressed by activin on the right
- Conjoined twins: right side of left twin affects left side of right twin
IV. Other
A) Holoprosencephaly
- Shh is needed for midline patterning in anterior (head)
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