92
Brain Development
Truncated Type II
receptor
TGF-
b
(dimer)
Type II receptor
Type I receptor
Smad2/3
Smad4
Overexpression of the truncated
type I receptors could act as
dominate negatives
P
P
P
P
P
(a)
WNT
WNT
+
LRP
LRP
Frizzled
Frizzled
Inactive
dishevelled
Activated
dishevelled
active
GSK-3
β
Inactive
GSK-3
β
Axin
Axin
APC
APC
Unphosphorylated
b
-catenin (stable)
Phosphorylated
b
-catenin (Unstable)
Translocated
into nucleus
Degraded
in proteasomes
Groucho
Groucho
LEF-1
/TCF
LEF-1
/TCF
β
-catenin
(b)
Wnt
P
Fig. 2
(a) TGF-
β
signaling machineries. (b) The canonical Wnt signaling
pathway.
neural tube therefore corresponds to the
mediolateral axis in the early neural plate:
both lateral ends of the neural plate, named
the
neural ridge
, become the dorsalmost
portion of the neural tube, while the
midline of the neural plate, which overlies
the axial mesoderm notochord, becomes
the ventralmost portion of the neural
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