31
Maternal mRNAs and Cell Lineages
Foxi2, encoded by a transcript enriched animally in
the oocyte, was identifed as a maternal regulator of foxi1
through upstream enhancer analysis (Cha et al., 2012).
Maternal mRNA depletion of foxi2 showed dysregulation of
foxi1 expression and gastrulation/axis defects but no major
loss of ectoderm derivatives or adhesion. Other animally
enriched maternal transcriptional regulators include Sox3,
which is thought to inhibit nodal5 expression (Zhang et al.,
2003; Zhang and Klymkowsky, 2007); Trim33, a putative
inhibitor of Smad4-dependent Tgfβ signaling ( Dupont et al.,
2005); and Znf585b, a Tp53 antagonist (Sasai et al., 2008).
A number of other molecules that can antagonize mesendoderm are also enriched animally (i.e. dand5, Bates et al.,
2013). These proteins might act to protect early ectoderm
fate (Reich and Weinstein, 2019).
Ectoderm development also involves homologues of
the mammalian pluripotency-related transcription factor
Pou5f1, including a maternal paralogue (pou5f3.3 [née
xlpou60/oct60]). Morpholino-based depletion of these
genes (singly or together) in Xenopus have implicated this
family in the maintenance of pluripotent character in the
animal cap (Morrison and Brickman, 2006) and in repression of Nodal/Tgfβ activity (Cao et al., 2006; Snir et al.,
2006). Additionally, Pou5f1 paralogs are required for
ectodermal adhesion and gastrulation, likely through the
activation of certain conserved Pou5f target genes, notably
lhx5 (see previously), sall1, and cdx1 (Livigni et al., 2013).
Maternal Pou5f3 (along with Sox3) has been shown to
establish competent chromatin prior to the onset of zygotic
transcription (Gentsch et al., 2019), suggesting the possibility that the maternally programmed early chromatin state
may form the basis for the ectoderm “default state.”
3.4.4. β-CATENIN IN DORSAL GENE ACTIVATION
In addition to regulating the timing of nodal expression,
β-Catenin regulates a number of dorsal genes directly,
including sia1, nodal3 paralogs, noggin, and chordin
(these last two are induced by β-Catenin but stabilized
by Nodal), that act in multiple aspects of organizer function. β-Catenin mainly regulates gene expression by derepressing TCF proteins and recruiting coactivators in the
context of Wnt-regulated enhancer complexes (Gammons
and Bienz, 2018). Both activities are supported by maternal
loss-of-function experiments in Xenopus (tcf7l1: Houston
et al., 2002; pygo2 and bcl9: Belenkaya et al., 2002;
Kennedy et al., 2010). Other maternal TCF proteins, Tcf7
and Tcf7l2, are also involved, with context-dependent activating and repressing roles on β-Catenin target genes (Roel
et al., 2003; Standley et al., 2006). Genomic studies have
begun to identify more complex interactions of β -Catenin
with many other transcription factors (e.g. Nishita et al.,
2000; Sinner et al., 2004; Zorn et al., 1999; reviewed in
Abu-Remaileh et al., 2010).
More recent evidence suggests that maternal β -Catenin
regulates dorsal gene expression prior to major MZT in
part through an epigenetic “poising” mechanism, marking
organizer-specifc genes in dorsal morula nuclei for later
expression by the recruitment of the histone arginine
methyltransferase Prmt2 (Blythe et al., 2010). Similarly,
maternal Foxh1 is also involved in presetting chromatin
for later expression, largely targeting subsequent Nodal/
Smad2-regulated genes (Afouda et al., 2020; Charney et
al., 2017; Chiu et al., 2014; Gupta et al., 2014). Foxh1 overlaps β-Catenin targets to a signifcant extent in these cases,
potentially acting as a “pioneer” factor to activate the opening of the chromatin conformation ( Zaret and Carroll, 2011).
3.4.5. CYTOPLASMIC ACTIVATION OF DORSAL β-CATENIN
Wnt/β-Catenin activation is dependent on cortical rotation and is most sensitive to induced stimulation or inhibition around the 16–32-cell stage (Kao et al., 1986; Yang
et al., 2002). Elinson, Gerhart, Kirschner, and others began
reinvestigating the cortical rotation phenomenon in the
1980s, ultimately leading to the characterization of dorsally
directed parallel microtubule array assembly in the vegetal
sub-cortical cytoplasm and kinesin-based translocation as
the underlying mechanism of cortical rotation (Gerhart et al.,
1989; Houston, 2017, 2012; Weaver and Kimelman, 2004).
The molecular control of cortical rotation is not well
understood. In addition to an overall increase in microtubule
nucleation and assembly following fertilization, likely related
to the cell cycle (Elinson, 1985; Olson et al., 2015), the ubiquitin ligase activity of Trim36, encoded by a vegetally localized mRNA is essential for normal parallel microtubule array
assembly and cortical rotation (Cuykendall and Houston,
2009). Another localized mRNA product, the Dnd1 RNAbinding protein (typically involved in germline specif cation),
is required to anchor trim36 in the cortex (Mei et al., 2013),
likely enriching Trim36 at the site of cortical rotation. The
targets of Trim36 ubiquitylation remain unknown. mRNA
depletion of vegetally localized plin2 also leads to defects in
cortical microtubule assembly during cortical rotation (Chan
et al., 2007), although it is unclear if the protein product
(Perilipin 2 is localized to lipid droplets) or the mRNA itself
are the relevant moiety (Kloc, 2009).
Cytoplasmic ablation/transplantation experiments have
generally supported the idea that cortical rotation dorsally
displaces a potent axis-inducing activity (Holowacz and
Elinson, 1993; Kageura, 1997). The identity of this activity remains unclear. Data suggest the vegetal cortical cytoplasm mimics Wnt activation and acts most similarly to
overexpressed Adenomatous Polyposis Coli (APC) protein
(Marikawa and Elinson, 1999). One caveat of these experiments is that the large size of the injected apc transcript
tends produce a variety of truncated/degradation products,
which may include dominant-inhibitory species (Vleminckx
et al., 1997 ).
Dishevelled and Frat1 (GBP) proteins are also candidates
for the cytoplasmic dorsalizing activity, based on the
visualization of “puncta” produced by injection of GFP
fusion constructs of these proteins and their potential for
dorsal translocation ( Miller et al., 1999; Weaver et al., 2003).
Maternal mRNAs and Cell Lineages
Foxi2, encoded by a transcript enriched animally in
the oocyte, was identifed as a maternal regulator of foxi1
through upstream enhancer analysis (Cha et al., 2012).
Maternal mRNA depletion of foxi2 showed dysregulation of
foxi1 expression and gastrulation/axis defects but no major
loss of ectoderm derivatives or adhesion. Other animally
enriched maternal transcriptional regulators include Sox3,
which is thought to inhibit nodal5 expression (Zhang et al.,
2003; Zhang and Klymkowsky, 2007); Trim33, a putative
inhibitor of Smad4-dependent Tgfβ signaling ( Dupont et al.,
2005); and Znf585b, a Tp53 antagonist (Sasai et al., 2008).
A number of other molecules that can antagonize mesendoderm are also enriched animally (i.e. dand5, Bates et al.,
2013). These proteins might act to protect early ectoderm
fate (Reich and Weinstein, 2019).
Ectoderm development also involves homologues of
the mammalian pluripotency-related transcription factor
Pou5f1, including a maternal paralogue (pou5f3.3 [née
xlpou60/oct60]). Morpholino-based depletion of these
genes (singly or together) in Xenopus have implicated this
family in the maintenance of pluripotent character in the
animal cap (Morrison and Brickman, 2006) and in repression of Nodal/Tgfβ activity (Cao et al., 2006; Snir et al.,
2006). Additionally, Pou5f1 paralogs are required for
ectodermal adhesion and gastrulation, likely through the
activation of certain conserved Pou5f target genes, notably
lhx5 (see previously), sall1, and cdx1 (Livigni et al., 2013).
Maternal Pou5f3 (along with Sox3) has been shown to
establish competent chromatin prior to the onset of zygotic
transcription (Gentsch et al., 2019), suggesting the possibility that the maternally programmed early chromatin state
may form the basis for the ectoderm “default state.”
3.4.4. β-CATENIN IN DORSAL GENE ACTIVATION
In addition to regulating the timing of nodal expression,
β-Catenin regulates a number of dorsal genes directly,
including sia1, nodal3 paralogs, noggin, and chordin
(these last two are induced by β-Catenin but stabilized
by Nodal), that act in multiple aspects of organizer function. β-Catenin mainly regulates gene expression by derepressing TCF proteins and recruiting coactivators in the
context of Wnt-regulated enhancer complexes (Gammons
and Bienz, 2018). Both activities are supported by maternal
loss-of-function experiments in Xenopus (tcf7l1: Houston
et al., 2002; pygo2 and bcl9: Belenkaya et al., 2002;
Kennedy et al., 2010). Other maternal TCF proteins, Tcf7
and Tcf7l2, are also involved, with context-dependent activating and repressing roles on β-Catenin target genes (Roel
et al., 2003; Standley et al., 2006). Genomic studies have
begun to identify more complex interactions of β -Catenin
with many other transcription factors (e.g. Nishita et al.,
2000; Sinner et al., 2004; Zorn et al., 1999; reviewed in
Abu-Remaileh et al., 2010).
More recent evidence suggests that maternal β -Catenin
regulates dorsal gene expression prior to major MZT in
part through an epigenetic “poising” mechanism, marking
organizer-specifc genes in dorsal morula nuclei for later
expression by the recruitment of the histone arginine
methyltransferase Prmt2 (Blythe et al., 2010). Similarly,
maternal Foxh1 is also involved in presetting chromatin
for later expression, largely targeting subsequent Nodal/
Smad2-regulated genes (Afouda et al., 2020; Charney et
al., 2017; Chiu et al., 2014; Gupta et al., 2014). Foxh1 overlaps β-Catenin targets to a signifcant extent in these cases,
potentially acting as a “pioneer” factor to activate the opening of the chromatin conformation ( Zaret and Carroll, 2011).
3.4.5. CYTOPLASMIC ACTIVATION OF DORSAL β-CATENIN
Wnt/β-Catenin activation is dependent on cortical rotation and is most sensitive to induced stimulation or inhibition around the 16–32-cell stage (Kao et al., 1986; Yang
et al., 2002). Elinson, Gerhart, Kirschner, and others began
reinvestigating the cortical rotation phenomenon in the
1980s, ultimately leading to the characterization of dorsally
directed parallel microtubule array assembly in the vegetal
sub-cortical cytoplasm and kinesin-based translocation as
the underlying mechanism of cortical rotation (Gerhart et al.,
1989; Houston, 2017, 2012; Weaver and Kimelman, 2004).
The molecular control of cortical rotation is not well
understood. In addition to an overall increase in microtubule
nucleation and assembly following fertilization, likely related
to the cell cycle (Elinson, 1985; Olson et al., 2015), the ubiquitin ligase activity of Trim36, encoded by a vegetally localized mRNA is essential for normal parallel microtubule array
assembly and cortical rotation (Cuykendall and Houston,
2009). Another localized mRNA product, the Dnd1 RNAbinding protein (typically involved in germline specif cation),
is required to anchor trim36 in the cortex (Mei et al., 2013),
likely enriching Trim36 at the site of cortical rotation. The
targets of Trim36 ubiquitylation remain unknown. mRNA
depletion of vegetally localized plin2 also leads to defects in
cortical microtubule assembly during cortical rotation (Chan
et al., 2007), although it is unclear if the protein product
(Perilipin 2 is localized to lipid droplets) or the mRNA itself
are the relevant moiety (Kloc, 2009).
Cytoplasmic ablation/transplantation experiments have
generally supported the idea that cortical rotation dorsally
displaces a potent axis-inducing activity (Holowacz and
Elinson, 1993; Kageura, 1997). The identity of this activity remains unclear. Data suggest the vegetal cortical cytoplasm mimics Wnt activation and acts most similarly to
overexpressed Adenomatous Polyposis Coli (APC) protein
(Marikawa and Elinson, 1999). One caveat of these experiments is that the large size of the injected apc transcript
tends produce a variety of truncated/degradation products,
which may include dominant-inhibitory species (Vleminckx
et al., 1997 ).
Dishevelled and Frat1 (GBP) proteins are also candidates
for the cytoplasmic dorsalizing activity, based on the
visualization of “puncta” produced by injection of GFP
fusion constructs of these proteins and their potential for
dorsal translocation ( Miller et al., 1999; Weaver et al., 2003).