Vertebrate Development Maternal to Zygotic Control (Advances in Experimental Medicine and Biology)

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species-specific. Recent data suggest that calcium transients downstream of the
chemokine receptor Ccr7 GPCR signaling are also involved in suppressing beta-
catenin activity (Wu et al. 2012b). The prominence of calcium regulation mecha-
nisms suggests tight control over beta-catenin stabilization in the zebrafish blastula.

6.3.4 Beta-Catenin Activity Dorsalizes the Primary Germ

Layers

Beta-catenin stabilization in the blastula following cortical rotation is the central
mechanism for establishing early dorsal fates across all three primary germ layers
(Fig. 6.8). In the vegetal prospective endodermal cells, beta-catenin is critical for the
dynamic regulation of Nodal expression and signaling, which is directly involved in
mesendoderm induction and patterning and likely constitutes what is referred to as
the Nieuwkoop center (see Chap. 7 ). In Xenopus, the typical nodal homolog genes
(e.g., nodal homologs 1, 2 , 4 , 5 , and 6 ) are expressed in a temporally and spatially

Cortical rotation Wnt activity

vd

nuclear beta-catenin

Nodal + Wnt activity

miR15/16

• wnt11b RNA


• Lrp6 signalosomes


• Dvl particles


• APC regulation?

nuclear beta-catenin

dYSL

Sfrp1; Frzb

Wnt5bCcr7

Vegt

sia1, nodal3.1

nodal5, 6

gsc, chrd

Vegt+beta-catenin

? bmp4

dharma bmp2b

nodal1

Eomes+beta-cateninEomes nodal1,2

gsc, chrd

blastoderm

blastocoel

(H3R8 diMe)

abc

Fig. 6.8 Models for Wnt/beta-catenin activation in Xenopus and zebrafish. (a) During cortical
rotation in Xenopus (top) and zebrafish (bottom), beta-catenin stabilizing dorsalizing activity is
transported into the equatorial region of the embryo by microtubule-mediated rotation of the cortex
and through transport along microtubule arrays. Candidates for this activity include wnt11b and
Lrp6/Dvl particles in Xenopus and wnt8a in zebrafish. (b) By the cleavage stages (16–128-cell
stage), beta-catenin be-comes activated and enriched in dorsal vegetal and marginal nuclei until
MBT. In Xenopus, priming of Wnt target genes occurs through dimethylation of Histone3 at argi-
nine 8 (H3R8). In zebrafish, beta-catenin accumulates in dorsal marginal and dYSL nuclei, and is
antagonized by multiple antagonists and calcium signaling mediators. (c) During the peri-MBT
stages, beta-catenin activates direct Wnt targets and cooperates with maternal T-domain proteins
(Vegt, Eomes) to activate nodal initially on the dorsal side. The combination of nodal and BMP
antagonism induced by beta-catenin induce the formation of the organizer (gsc, chrd)

D.W. Houston

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