32
Xenopus
However, the localization of the endogenous proteins is not
known. Furthermore, maternal Dvl2/3 depletion results in
dorsalized embryos, opposite to expectation (Tadjuidje et al.,
2011), and Frat1, although required for axis formation in
Xenopus (Yost et al., 1998), may not actually function generally in Wnt/β-Catenin regulation (Amerongen et al., 2010).
Recent work on a spontaneous ventralized zebraf sh
maternal-effect mutation huluwa (hwa; Yan et al., 2018)
has shown that the novel Hwa protein may act to promote
the dorsal degradation of Axin1, a key negative regulator of
β-Catenin. Hwa is encoded by a localized maternal mRNA
in both fsh and frogs and is a novel membrane protein
that accumulates dorsally in the blastula (Yan et al., 2018).
Dorsal down-regulation of Axin1 had been inferred from
experiments showing the reduced ability of dorsally injected
axin1 mRNA to rescue maternal axin1 depletion compared
to ventrally injected axin1 (Kofron et al., 2001).
3.4.6. SECRETED LIGAND ACTIVATION OF β-CATENIN
In contrast to the idea of strict intracellular activation of
dorsal Wnt/β-Catenin by cytoplasmic determinants outlined
previously, current models suggest that maternal Wnt11b
ligand, encoded by a vegetally localized mRNA, is the
main determinant for axis specifcation in Xenopus. The
observation that overexpressed extracellular Wnt antagonists
fail to inhibit endogenous Wnt/β-Catenin activity or axis
determination tends to support the intracellular model
(Hoppler et al., 1996; Leyns et al., 1997; Wang et al., 1997;
Yan et al., 2018). However, antisense depletion experiments
also demonstrate that inhibition of maternal wnt11b, encoding a secreted ligand, does result in ventralization and in
reduced dorsal β-Catenin activity (Tao et al., 2005).
Although Wnt11b is commonly classifed as a “noncanonical” Wnt ligand (non-β-Catenin-activating), there is
a body of evidence pointing toward receptor/co-receptor
context-dependent regulation of β-Catenin-dependent and -
independent pathways by Wnt11 proteins and other Wnts
(e.g. He et al., 1995). The timing of Wnt11b action is not
known, but it must act prior to the 16-cell stage (see previously). The extent and mechanisms by which Wnt11b
activity would become enriched dorsally by cortical rotation
is also unclear, and recent transcriptomic data from single
cleavage-stage blastomeres ( X. laevis: Flachsova et al., 2013;
X. tropicalis: Collart et al., 2014; Domenico et al., 2015)
suggest little to no dorsal enrichment of wnt11b mRNA or
wnt11b polyadenylation.
Maternal mRNA depletion studies have indirectly
implicated translational regulation, potentially targeting
wnt11b, in dorsal signaling. The RNA-binding protein Bicc1
is encoded by a localized maternal mRNA in Xenopus
(Wessely and Robertis, 2000) and can bind directly to the
3’UTRs of dand5, tdgf1.3, and wnt11b and repress their
translation in reporter assays (Park et al., 2016; Zhang
et al., 2013). Depletion of maternal bicc1 leads to dorsoanteriorized embryos, suggesting that translational repression, mediated by RNA-binding KH domains (Dowdle et al.,
2019; Park et al., 2016), is necessary to restrict the activities of one or more of these or other molecules. Bicc1 was
f rst identifed in a screen for mRNAs with reduced polyadenylation in the embryos ventralized by UV-irradiation
(Wessely and Robertis, 2000). One implication of this result
is that bicc1 polyadenylation, and hence Bicc1 protein,
might be higher dorsally. However, this situation would predict that Bicc1 should normally inhibit wnt11b translation
dorsally. In the absence of Bicc1, overactive Wnt11b signals
might be involved in dorsalizing the embryo, but it remains
unclear whether this mechanism controls endogenous dorsal
β -Catenin accumulation.
One proposed synthesis of these ideas is that Wnt signaling may be potentiated by the endocytosis of activated
Wnt receptor-coreceptor complexes (“Lrp6 signalosomes”)
in association with Dvl (Bilic et al., 2007). Cortical rotation might thus enrich Wnt signalosomes on the dorsal side
(Dobrowolski and Robertis, 2012), activating dorsal signaling through the perpetuation of an earlier signaling event or
by possibly sequestering β -Catenin degradation machinery
(e.g. Gsk3b) in multi-vesicular bodies (Taelman et al., 2010).
However, recent data from cell-line experiments suggest
that endocytosis of Wnt receptor-co-receptor complexes is
not required for subsequent signal transduction (Rim et al.,
2020), and the extent that signalosomes differentially accumulate dorsally remains unknown.
While genetic studies in mice have shown that β -Catenin
signaling is required for anterior visceral endoderm (AVE)
formation (the key step in mammalian axial patterning), it
has become clear that secreted Wnt activity is dispensable in
this regard (reviewed in Houston, 2017). Similar to the case
in Xenopus, Tdgf1-mediated signals are required upstream
of β-Catenin stabilization in the AVE (Morkel et al., 2003),
but the extent to which this similarity represents a coincidental convergence of signals or a deeper level of conservation in mechanisms for establishing bilateral symmetry
remains unclear.
3.5. MATERNAL CONTROL OF PRIMORDIAL
GERM CELL FORMATION
The idea of a separate germline and its specifcation by localized maternal determinants arose from classic studies in
insects, following the observation of a conspicuous budding
of primordial germ cells from the posterior pole of the early
blastoderm (classical literature reviewed in Hegner, 1914).
Additional evidence for what came to be called “germ plasm”
was subsequently revealed in many other organisms (Beams
and Kessel, 1974; Eddy, 1975; Extavour and Akam, 2003).
3.5.1. THE GERM PLASM
The identif cation of germ plasm in a vertebrate embryo (in
Rana frogs, Bounoure, 1934, 1931) suggested that cytoplasmic inheritance of germline fate might represent a general
mechanism. Other evidence from salamanders, mammals,
and other organisms however suggested that primordial
Xenopus
However, the localization of the endogenous proteins is not
known. Furthermore, maternal Dvl2/3 depletion results in
dorsalized embryos, opposite to expectation (Tadjuidje et al.,
2011), and Frat1, although required for axis formation in
Xenopus (Yost et al., 1998), may not actually function generally in Wnt/β-Catenin regulation (Amerongen et al., 2010).
Recent work on a spontaneous ventralized zebraf sh
maternal-effect mutation huluwa (hwa; Yan et al., 2018)
has shown that the novel Hwa protein may act to promote
the dorsal degradation of Axin1, a key negative regulator of
β-Catenin. Hwa is encoded by a localized maternal mRNA
in both fsh and frogs and is a novel membrane protein
that accumulates dorsally in the blastula (Yan et al., 2018).
Dorsal down-regulation of Axin1 had been inferred from
experiments showing the reduced ability of dorsally injected
axin1 mRNA to rescue maternal axin1 depletion compared
to ventrally injected axin1 (Kofron et al., 2001).
3.4.6. SECRETED LIGAND ACTIVATION OF β-CATENIN
In contrast to the idea of strict intracellular activation of
dorsal Wnt/β-Catenin by cytoplasmic determinants outlined
previously, current models suggest that maternal Wnt11b
ligand, encoded by a vegetally localized mRNA, is the
main determinant for axis specifcation in Xenopus. The
observation that overexpressed extracellular Wnt antagonists
fail to inhibit endogenous Wnt/β-Catenin activity or axis
determination tends to support the intracellular model
(Hoppler et al., 1996; Leyns et al., 1997; Wang et al., 1997;
Yan et al., 2018). However, antisense depletion experiments
also demonstrate that inhibition of maternal wnt11b, encoding a secreted ligand, does result in ventralization and in
reduced dorsal β-Catenin activity (Tao et al., 2005).
Although Wnt11b is commonly classifed as a “noncanonical” Wnt ligand (non-β-Catenin-activating), there is
a body of evidence pointing toward receptor/co-receptor
context-dependent regulation of β-Catenin-dependent and -
independent pathways by Wnt11 proteins and other Wnts
(e.g. He et al., 1995). The timing of Wnt11b action is not
known, but it must act prior to the 16-cell stage (see previously). The extent and mechanisms by which Wnt11b
activity would become enriched dorsally by cortical rotation
is also unclear, and recent transcriptomic data from single
cleavage-stage blastomeres ( X. laevis: Flachsova et al., 2013;
X. tropicalis: Collart et al., 2014; Domenico et al., 2015)
suggest little to no dorsal enrichment of wnt11b mRNA or
wnt11b polyadenylation.
Maternal mRNA depletion studies have indirectly
implicated translational regulation, potentially targeting
wnt11b, in dorsal signaling. The RNA-binding protein Bicc1
is encoded by a localized maternal mRNA in Xenopus
(Wessely and Robertis, 2000) and can bind directly to the
3’UTRs of dand5, tdgf1.3, and wnt11b and repress their
translation in reporter assays (Park et al., 2016; Zhang
et al., 2013). Depletion of maternal bicc1 leads to dorsoanteriorized embryos, suggesting that translational repression, mediated by RNA-binding KH domains (Dowdle et al.,
2019; Park et al., 2016), is necessary to restrict the activities of one or more of these or other molecules. Bicc1 was
f rst identifed in a screen for mRNAs with reduced polyadenylation in the embryos ventralized by UV-irradiation
(Wessely and Robertis, 2000). One implication of this result
is that bicc1 polyadenylation, and hence Bicc1 protein,
might be higher dorsally. However, this situation would predict that Bicc1 should normally inhibit wnt11b translation
dorsally. In the absence of Bicc1, overactive Wnt11b signals
might be involved in dorsalizing the embryo, but it remains
unclear whether this mechanism controls endogenous dorsal
β -Catenin accumulation.
One proposed synthesis of these ideas is that Wnt signaling may be potentiated by the endocytosis of activated
Wnt receptor-coreceptor complexes (“Lrp6 signalosomes”)
in association with Dvl (Bilic et al., 2007). Cortical rotation might thus enrich Wnt signalosomes on the dorsal side
(Dobrowolski and Robertis, 2012), activating dorsal signaling through the perpetuation of an earlier signaling event or
by possibly sequestering β -Catenin degradation machinery
(e.g. Gsk3b) in multi-vesicular bodies (Taelman et al., 2010).
However, recent data from cell-line experiments suggest
that endocytosis of Wnt receptor-co-receptor complexes is
not required for subsequent signal transduction (Rim et al.,
2020), and the extent that signalosomes differentially accumulate dorsally remains unknown.
While genetic studies in mice have shown that β -Catenin
signaling is required for anterior visceral endoderm (AVE)
formation (the key step in mammalian axial patterning), it
has become clear that secreted Wnt activity is dispensable in
this regard (reviewed in Houston, 2017). Similar to the case
in Xenopus, Tdgf1-mediated signals are required upstream
of β-Catenin stabilization in the AVE (Morkel et al., 2003),
but the extent to which this similarity represents a coincidental convergence of signals or a deeper level of conservation in mechanisms for establishing bilateral symmetry
remains unclear.
3.5. MATERNAL CONTROL OF PRIMORDIAL
GERM CELL FORMATION
The idea of a separate germline and its specifcation by localized maternal determinants arose from classic studies in
insects, following the observation of a conspicuous budding
of primordial germ cells from the posterior pole of the early
blastoderm (classical literature reviewed in Hegner, 1914).
Additional evidence for what came to be called “germ plasm”
was subsequently revealed in many other organisms (Beams
and Kessel, 1974; Eddy, 1975; Extavour and Akam, 2003).
3.5.1. THE GERM PLASM
The identif cation of germ plasm in a vertebrate embryo (in
Rana frogs, Bounoure, 1934, 1931) suggested that cytoplasmic inheritance of germline fate might represent a general
mechanism. Other evidence from salamanders, mammals,
and other organisms however suggested that primordial