98
Xenopus
and Hes5.1 reduced hes7.3/esr5 induction by Nodal2.
Therefore, it was proposed that a mutually antagonistic relationship between hes5.1 and hes7.3/esr5 controls the balance
of mesoderm specifcation within the IMZ (Kinoshita et al.,
2011). It will be interesting to determine whether hes5.1 also
mediates endodermal versus mesodermal choices.
hes5.5 and hes5.6 are positively regulated by canonical
Notch-RBPJ in the IMZ (Miazga and McLaughlin, 2009).
While the function of hes5.5 in this tissue has not been
studied, Notch regulates the timing of heart f eld specif cation, possibly through hes5.6 ( Tables 7.5 , 7.6 ) ( Miazga and
McLaughlin, 2009). While hes6.1 is negatively regulated
by Notch/RBPJ in the neural plate, it is not known whether
this pathway controls hes6.1 expression in the IMZ during
gastrulation, although it is a direct Wnt/β-Catenin target
and requires input from zygotic Wnt/β-Catenin signaling
(Hufton et al., 2006; Kjolby and Harland, 2017). hes6.1
favors paraxial mesoderm development (but not general
mesoderm induction) by sequestering TLE/Groucho corepressors, thus relieving myod1 from repression in mesodermal precursors (Cossins et al., 2002; Murai et al., 2007)
( Table 7.6 ).
7.3.3. DORSAL MIDLINE TISSUES
Cells that derive from the dorsal MZ/organizer region constitute the vertebrate dorsal midline (DML), an essential
signaling center for development of the surrounding tissues.
The DML gives rise to several derivatives: (1) the prechordal
endomesoderm (PEM), a key signaling center for anterior
neural development that emerges from the deep cells of
the organizer to form the prechordal plate; (2) the notoplate (Figure 7.3B, D), which gradually converges and extends
during gastrulation to form the foor plate (FP) of the neural
tube; (3) the notochord; and (4) the dorsal midline of the
endoderm, in Xenopus known as the gastrocoel roof plate
(GRP), which functions as a left-right organizer. During neurulation, some GRP cells incorporate into the notochord and
somites, while bilateral GRP rows gradually fuse into the
hypochord, ventral to the notochord (Keller and Danilchik,
1988; Minsuk and Keller, 1997; Kiecker and Niehrs, 2001;
Shook et al., 2004; López and Carrasco, 2006) (Figure
7.3D). Gene marker studies revealed that the precursors of
these various derivatives are intermingled at the beginning
of gastrulation but gradually segregate (Bouwmeester et al.,
1996; Artinger et al., 1997; Yamaguti et al., 2005); this process is highly infuenced by the Notch pathway.
Components of the Notch pathway are differentially
expressed in the multipotent DML precursors that either
involute (as the IMZ) or remain on the surface as the NIMZ,
that is, notoplate (López et al., 2003; López et al., 2005).
hes4 is expressed in the dorsal NIMZ and then in the notoplate and FP. dll1 is expressed in a compact domain throughout the IMZ, except for the organizer region, where only
scattered cells express dll1 and also hes4 prior to involution.
Once these dorsal IMZ cells involute, only hes4 expression
continues, restricted to the prechordal mesoderm but absent
from the notochord. In fact, hes4 is the only hes1–7 gene
expressed in the Xenopus DML during gastrulation and neurulation (Figure 7.2) (Tsuji et al., 2003; López et al., 2005;
Yamaguti et al., 2005).
We perturbed the Notch pathway in several ways to
address its role during DML development, including hes4
overexpression and knock-down, constitutive NICD1 activation, time-controlled GR-NICD1 activation, blocking the
whole notch1 pathway by knock-down, the RBPJ-dependent
pathway with RBPJ DBM, Dll1 signaling with dll1 STU, and
Notch processing with psen1 knock-down. Our results indicated that during gastrulation, notch1/psen1/RBPJ/hes4
signaling favors notoplate over notochord fate (López et al.,
2003; López et al., 2005) (Tables 7.5, 7.6 ). Other authors
showed that Notch promotes hypochord over notochord by
injecting NICD1 and RBPJ DBM ( Peyrot et al., 2011) ( Table
7.5). As the PEM mesodermal population segregates during gastrulation into two subdomains, hes4 and gsc are
expressed in the anterior prechordal mesoderm (aPM),
whereas chordin is expressed in the posterior prechordal
mesoderm (pPM) (Yamaguti et al., 2005). It was proposed
that hes4 initially ensures an organizer environment by
inducing early organizer genes through a non-cell-autonomous activity that depends on the WRPW domain. Then,
hes4 is required for aPM specifcation, as it inhibits contiguous fates through a cell-autonomous repressive activity, restricting pPM and notochord (Yamaguti et al., 2005;
Murato et al., 2006) (Table 7.5).
Based on these studies, we propose a model for how the
Notch pathway allocates dorsal MZ descendants into the
different DML tissues (Figure 7.3B, D). First, DML precursors choose between aPM or pPM fates. Dll1 from scattered
cells in the organizer induces hes4 in neighboring cells,
which represses pPM fates, thus promoting aPM. As gastrulation proceeds and more posterior cells involute, multipotent precursors in the mid- and late organizer choose
between FP, notochord, or hypochord fates. Dll1 from scattered cells in the boundary between the dorsal NIMZ and
the pre-involuted IMZ interacts with the Notch1 receptor on
the surrounding cells, activating hes4 to repress the genes
that promote notochord development and impede their involution so they gradually incorporate into the notoplate. By
this mechanism, dll1 executes a cell fate switch that favors
notoplate development at the expense of notochord. Dll1
presented by the IMZ cells fanking the organizer activate
Notch1 signaling in a pair of bilateral rows of dorsal IMZ
cells, favoring hypochord over notochord; the down-stream
mechanism is unknown, since hes4 is not expressed by
hypochord precursors. In addition, notch1/hes4 expand the
expression of foxa4, a positive notoplate/FP regulator (López
et al., 2003; López et al., 2005), whereas foxa4 knock-down
suppress hes4 in the FP (Murgan et al., 2014), suggesting
they establish a positive feedback loop. The expression of
hey1, which is positively regulated by Notch/RBPJ (Pichon
et al., 2002; Rones et al., 2002) (Tables 7.2, 7.3), matches the
time and spatial profle of hypochord development, with initial bilateral stripes in the GRP that later fuse at the midline
Xenopus
and Hes5.1 reduced hes7.3/esr5 induction by Nodal2.
Therefore, it was proposed that a mutually antagonistic relationship between hes5.1 and hes7.3/esr5 controls the balance
of mesoderm specifcation within the IMZ (Kinoshita et al.,
2011). It will be interesting to determine whether hes5.1 also
mediates endodermal versus mesodermal choices.
hes5.5 and hes5.6 are positively regulated by canonical
Notch-RBPJ in the IMZ (Miazga and McLaughlin, 2009).
While the function of hes5.5 in this tissue has not been
studied, Notch regulates the timing of heart f eld specif cation, possibly through hes5.6 ( Tables 7.5 , 7.6 ) ( Miazga and
McLaughlin, 2009). While hes6.1 is negatively regulated
by Notch/RBPJ in the neural plate, it is not known whether
this pathway controls hes6.1 expression in the IMZ during
gastrulation, although it is a direct Wnt/β-Catenin target
and requires input from zygotic Wnt/β-Catenin signaling
(Hufton et al., 2006; Kjolby and Harland, 2017). hes6.1
favors paraxial mesoderm development (but not general
mesoderm induction) by sequestering TLE/Groucho corepressors, thus relieving myod1 from repression in mesodermal precursors (Cossins et al., 2002; Murai et al., 2007)
( Table 7.6 ).
7.3.3. DORSAL MIDLINE TISSUES
Cells that derive from the dorsal MZ/organizer region constitute the vertebrate dorsal midline (DML), an essential
signaling center for development of the surrounding tissues.
The DML gives rise to several derivatives: (1) the prechordal
endomesoderm (PEM), a key signaling center for anterior
neural development that emerges from the deep cells of
the organizer to form the prechordal plate; (2) the notoplate (Figure 7.3B, D), which gradually converges and extends
during gastrulation to form the foor plate (FP) of the neural
tube; (3) the notochord; and (4) the dorsal midline of the
endoderm, in Xenopus known as the gastrocoel roof plate
(GRP), which functions as a left-right organizer. During neurulation, some GRP cells incorporate into the notochord and
somites, while bilateral GRP rows gradually fuse into the
hypochord, ventral to the notochord (Keller and Danilchik,
1988; Minsuk and Keller, 1997; Kiecker and Niehrs, 2001;
Shook et al., 2004; López and Carrasco, 2006) (Figure
7.3D). Gene marker studies revealed that the precursors of
these various derivatives are intermingled at the beginning
of gastrulation but gradually segregate (Bouwmeester et al.,
1996; Artinger et al., 1997; Yamaguti et al., 2005); this process is highly infuenced by the Notch pathway.
Components of the Notch pathway are differentially
expressed in the multipotent DML precursors that either
involute (as the IMZ) or remain on the surface as the NIMZ,
that is, notoplate (López et al., 2003; López et al., 2005).
hes4 is expressed in the dorsal NIMZ and then in the notoplate and FP. dll1 is expressed in a compact domain throughout the IMZ, except for the organizer region, where only
scattered cells express dll1 and also hes4 prior to involution.
Once these dorsal IMZ cells involute, only hes4 expression
continues, restricted to the prechordal mesoderm but absent
from the notochord. In fact, hes4 is the only hes1–7 gene
expressed in the Xenopus DML during gastrulation and neurulation (Figure 7.2) (Tsuji et al., 2003; López et al., 2005;
Yamaguti et al., 2005).
We perturbed the Notch pathway in several ways to
address its role during DML development, including hes4
overexpression and knock-down, constitutive NICD1 activation, time-controlled GR-NICD1 activation, blocking the
whole notch1 pathway by knock-down, the RBPJ-dependent
pathway with RBPJ DBM, Dll1 signaling with dll1 STU, and
Notch processing with psen1 knock-down. Our results indicated that during gastrulation, notch1/psen1/RBPJ/hes4
signaling favors notoplate over notochord fate (López et al.,
2003; López et al., 2005) (Tables 7.5, 7.6 ). Other authors
showed that Notch promotes hypochord over notochord by
injecting NICD1 and RBPJ DBM ( Peyrot et al., 2011) ( Table
7.5). As the PEM mesodermal population segregates during gastrulation into two subdomains, hes4 and gsc are
expressed in the anterior prechordal mesoderm (aPM),
whereas chordin is expressed in the posterior prechordal
mesoderm (pPM) (Yamaguti et al., 2005). It was proposed
that hes4 initially ensures an organizer environment by
inducing early organizer genes through a non-cell-autonomous activity that depends on the WRPW domain. Then,
hes4 is required for aPM specifcation, as it inhibits contiguous fates through a cell-autonomous repressive activity, restricting pPM and notochord (Yamaguti et al., 2005;
Murato et al., 2006) (Table 7.5).
Based on these studies, we propose a model for how the
Notch pathway allocates dorsal MZ descendants into the
different DML tissues (Figure 7.3B, D). First, DML precursors choose between aPM or pPM fates. Dll1 from scattered
cells in the organizer induces hes4 in neighboring cells,
which represses pPM fates, thus promoting aPM. As gastrulation proceeds and more posterior cells involute, multipotent precursors in the mid- and late organizer choose
between FP, notochord, or hypochord fates. Dll1 from scattered cells in the boundary between the dorsal NIMZ and
the pre-involuted IMZ interacts with the Notch1 receptor on
the surrounding cells, activating hes4 to repress the genes
that promote notochord development and impede their involution so they gradually incorporate into the notoplate. By
this mechanism, dll1 executes a cell fate switch that favors
notoplate development at the expense of notochord. Dll1
presented by the IMZ cells fanking the organizer activate
Notch1 signaling in a pair of bilateral rows of dorsal IMZ
cells, favoring hypochord over notochord; the down-stream
mechanism is unknown, since hes4 is not expressed by
hypochord precursors. In addition, notch1/hes4 expand the
expression of foxa4, a positive notoplate/FP regulator (López
et al., 2003; López et al., 2005), whereas foxa4 knock-down
suppress hes4 in the FP (Murgan et al., 2014), suggesting
they establish a positive feedback loop. The expression of
hey1, which is positively regulated by Notch/RBPJ (Pichon
et al., 2002; Rones et al., 2002) (Tables 7.2, 7.3), matches the
time and spatial profle of hypochord development, with initial bilateral stripes in the GRP that later fuse at the midline