111
Notch Signaling in Early Embryogenesis
negative feedback loop. Cell oscillations slow down towards
the anterior presomitic mesoderm, generating a kinematic
wave of cycling gene expression. Opposite gradients of RA
from the anterior presomitic mesoderm and FGF and Wnt
signaling from the tailbud region defne a so-called “determination wavefront,” which sweeps through the presomitic
mesoderm rostro-caudally. When cycling cells at the permissive phase are reached by the determination wavefront,
they stop oscillating. This results in the striped activation
of mesp genes. Consequently, the anterior presomitic mesoderm forms whorls of “somitomeres” whose gene expression
prepattern delineates the future boundaries that lead to the
formation of individual somites (Figure 7.1D). During vertebrate somitogenesis, intercellular Dll/Notch signaling acts
frst in the posterior presomitic mesoderm to synchronize the
frequencies of neighboring cell-oscillators and then in the
anterior presomitic mesoderm to position the future intersomitic boundary and def ne the anterior-posterior polarity of
the somite (Cooke, 1981; Takahashi et al., 2000; Pourquié
and Tam, 2001; Moreno and Kintner, 2004; Nagano et al.,
2006; Sparrow, 2008; Gomez et al., 2008; Sasaki et al., 2011;
Oates et al., 2012; Hubaud and Pourquié, 2014; Wahi et al.,
2016; Janesick et al., 2017; Venzin and Oates, 2020; Naoki
and Matsui, 2020).
7.3.6.1. Notch Ligands and Hes Genes in Somitogenesis
While the general segmentation mechanism is conserved
across vertebrates, the individual hes and delta oscillating
genes involved vary between species (Oates et al., 2012).
In Xenopus, several Notch pathway genes are expressed in
discrete stripes in the presomitic mesoderm. So far, only
dlc, hes5.3, hes5.5, and hes5.6 were reported as oscillatory in Xenopus (Table 7.12, Figure 7.1D). Since a large
number of embryos must be analyzed to discern a changing pattern that results from oscillatory expression (Figure
7.1D, lower panel), some genes with cycling behavior might
have been overlooked (Sparrow, 2008). Interestingly, dlc
delineates somitomeres at late gastrula (Peres et al., 2006),
signifcantly earlier than genes involved in somite segregation (Durston et al., 2018). Notably, hes4, hes5.3, hes5.6,
and dlc show a left-right asynchrony in their somitomeric
pattern (Davis et al., 2001; Li et al., 2003; Blewitt, 2009;
Durston et al., 2018) (Figure 7.1D, lower panel). A careful
examination led to the proposal that somitogenesis waves
are propagated as counter-clockwise spirals, probably
linked to the mechanisms imposing left-right asymmetries
(Durston et al., 2018).
The consequences of the experimental perturbation of
Dlc/Notch/RBPJ signaling in Xenopus are consistent with
a crucial role in regulating somitogenesis (Jen et al., 1997;
Sparrow et al., 1998; Peres et al., 2006) (Table 7.12). Although
dll1 is expressed in the tailbud in a poorly described segmental prepattern (Table 7.12), its possible role in somitogenesis
has been overlooked. notch1 shows continuous expression
throughout the tailbud presomitic mesoderm but is restricted
to one-half of mature somites, whereas jag2 is expressed in
the opposite pattern (Table 7.12), suggesting an interplay
between notch1 and jag2 after somite segregation.
Among hes genes for which there is a precise description
of their somitomeric expression pattern, hes4 is restricted
to posterior compartments, and others, including dlc, are
restricted to anterior compartments (Figure 7.1D) (Jen et al.,
1997 ). A paired RBPJ motif in the proximal promoter of
hes4, including the intervening hexamer, is necessary for its
somitomeric pattern together with its 3’UTR, which confers
mRNA instability except in its striped domains in the presomitic mesoderm. Since the 3’UTR of hes5.6 can impose
this striped pattern on hes4, cyclic hes5.6 expression might
also be regulated by mRNA decay (Davis et al., 2001). The
results summarized in Table 7.12 indicate that spatially controlled dlc expression is necessary for Xenopus somitogenesis and for setting the normal segmental prepattern of Notch
targets related to the segmentation program (hes7.2, hes7.3/
esr5, hes4). Notch/RBPJ represses dlc and mespa, whereas
Dlc from the anterior half of somitomeres activates hes4 in
the posterior half through Notch/RBPJ.
hes7.3/esr5 is necessary for proper somitogenesis, including the refnement of dlc, hes7.2, and mespa expression into
stripes in the so-called “transition zone” between the somitomeric and tailbud regions (Table 7.12) (Figure 7.1D) (Jen et
al., 1999). It was proposed that the Notch pathway uniformly
activates targets like hes7.3/esr5 in the tailbud region. Then,
a mechanism requiring de novo protein synthesis and HDAC
represses dlc, hes7.2, and hes7.3/esr5 in the transition zone,
which introduces an expression gap that generates an on/
off periodicity that is stably maintained in the somitomeres.
hes7.3/esr5 participates in a negative feedback loop, repressing dlc and hes7.2 in posterior half-segments in the transition zone from where somitomeres arise. In contrast, rostral
to the transition zone, hes7.3/esr5 participates in a positive
feedback loop, maintaining the segmental dlc prepattern in
the somitomeric region (Jen et al., 1999).
hes6.1 shows a broad tailbud expression domain and
a segmental prepattern in somitomeres (Table 7.12).
Overexpression of hes6.1 or a mutant DNA binding form
severely disrupted somitogenesis and molecular markers
(Cossins et al., 2002), suggesting that hes6.1 must be spatially regulated in the presomitic mesoderm for proper segmentation, perhaps by protein-protein interactions rather than
DNA binding. Interestingly, hes6.1 is negatively regulated by
Notch/RBPJ in the neural plate (Koyano-Nakagawa et al.,
2000), so it will be interesting to study a possible interplay
between hes6.1 and the Notch pathway in somitogenesis.
7.3.6.2. Interplay between Notch and Other
Genes and Pathways in Somitogenesis
Somite boundary formation is also regulated by the Notch
pathway via repression of protocadherin 8 (pcdh8) in the
posterior half of somitomeres. Pcdh8, which is expressed in
their anterior half, in turn regulates differential cell adhesion and prevents the intermingling of anterior and posterior cells between somitomeres, contributing to maintaining
Notch Signaling in Early Embryogenesis
negative feedback loop. Cell oscillations slow down towards
the anterior presomitic mesoderm, generating a kinematic
wave of cycling gene expression. Opposite gradients of RA
from the anterior presomitic mesoderm and FGF and Wnt
signaling from the tailbud region defne a so-called “determination wavefront,” which sweeps through the presomitic
mesoderm rostro-caudally. When cycling cells at the permissive phase are reached by the determination wavefront,
they stop oscillating. This results in the striped activation
of mesp genes. Consequently, the anterior presomitic mesoderm forms whorls of “somitomeres” whose gene expression
prepattern delineates the future boundaries that lead to the
formation of individual somites (Figure 7.1D). During vertebrate somitogenesis, intercellular Dll/Notch signaling acts
frst in the posterior presomitic mesoderm to synchronize the
frequencies of neighboring cell-oscillators and then in the
anterior presomitic mesoderm to position the future intersomitic boundary and def ne the anterior-posterior polarity of
the somite (Cooke, 1981; Takahashi et al., 2000; Pourquié
and Tam, 2001; Moreno and Kintner, 2004; Nagano et al.,
2006; Sparrow, 2008; Gomez et al., 2008; Sasaki et al., 2011;
Oates et al., 2012; Hubaud and Pourquié, 2014; Wahi et al.,
2016; Janesick et al., 2017; Venzin and Oates, 2020; Naoki
and Matsui, 2020).
7.3.6.1. Notch Ligands and Hes Genes in Somitogenesis
While the general segmentation mechanism is conserved
across vertebrates, the individual hes and delta oscillating
genes involved vary between species (Oates et al., 2012).
In Xenopus, several Notch pathway genes are expressed in
discrete stripes in the presomitic mesoderm. So far, only
dlc, hes5.3, hes5.5, and hes5.6 were reported as oscillatory in Xenopus (Table 7.12, Figure 7.1D). Since a large
number of embryos must be analyzed to discern a changing pattern that results from oscillatory expression (Figure
7.1D, lower panel), some genes with cycling behavior might
have been overlooked (Sparrow, 2008). Interestingly, dlc
delineates somitomeres at late gastrula (Peres et al., 2006),
signifcantly earlier than genes involved in somite segregation (Durston et al., 2018). Notably, hes4, hes5.3, hes5.6,
and dlc show a left-right asynchrony in their somitomeric
pattern (Davis et al., 2001; Li et al., 2003; Blewitt, 2009;
Durston et al., 2018) (Figure 7.1D, lower panel). A careful
examination led to the proposal that somitogenesis waves
are propagated as counter-clockwise spirals, probably
linked to the mechanisms imposing left-right asymmetries
(Durston et al., 2018).
The consequences of the experimental perturbation of
Dlc/Notch/RBPJ signaling in Xenopus are consistent with
a crucial role in regulating somitogenesis (Jen et al., 1997;
Sparrow et al., 1998; Peres et al., 2006) (Table 7.12). Although
dll1 is expressed in the tailbud in a poorly described segmental prepattern (Table 7.12), its possible role in somitogenesis
has been overlooked. notch1 shows continuous expression
throughout the tailbud presomitic mesoderm but is restricted
to one-half of mature somites, whereas jag2 is expressed in
the opposite pattern (Table 7.12), suggesting an interplay
between notch1 and jag2 after somite segregation.
Among hes genes for which there is a precise description
of their somitomeric expression pattern, hes4 is restricted
to posterior compartments, and others, including dlc, are
restricted to anterior compartments (Figure 7.1D) (Jen et al.,
1997 ). A paired RBPJ motif in the proximal promoter of
hes4, including the intervening hexamer, is necessary for its
somitomeric pattern together with its 3’UTR, which confers
mRNA instability except in its striped domains in the presomitic mesoderm. Since the 3’UTR of hes5.6 can impose
this striped pattern on hes4, cyclic hes5.6 expression might
also be regulated by mRNA decay (Davis et al., 2001). The
results summarized in Table 7.12 indicate that spatially controlled dlc expression is necessary for Xenopus somitogenesis and for setting the normal segmental prepattern of Notch
targets related to the segmentation program (hes7.2, hes7.3/
esr5, hes4). Notch/RBPJ represses dlc and mespa, whereas
Dlc from the anterior half of somitomeres activates hes4 in
the posterior half through Notch/RBPJ.
hes7.3/esr5 is necessary for proper somitogenesis, including the refnement of dlc, hes7.2, and mespa expression into
stripes in the so-called “transition zone” between the somitomeric and tailbud regions (Table 7.12) (Figure 7.1D) (Jen et
al., 1999). It was proposed that the Notch pathway uniformly
activates targets like hes7.3/esr5 in the tailbud region. Then,
a mechanism requiring de novo protein synthesis and HDAC
represses dlc, hes7.2, and hes7.3/esr5 in the transition zone,
which introduces an expression gap that generates an on/
off periodicity that is stably maintained in the somitomeres.
hes7.3/esr5 participates in a negative feedback loop, repressing dlc and hes7.2 in posterior half-segments in the transition zone from where somitomeres arise. In contrast, rostral
to the transition zone, hes7.3/esr5 participates in a positive
feedback loop, maintaining the segmental dlc prepattern in
the somitomeric region (Jen et al., 1999).
hes6.1 shows a broad tailbud expression domain and
a segmental prepattern in somitomeres (Table 7.12).
Overexpression of hes6.1 or a mutant DNA binding form
severely disrupted somitogenesis and molecular markers
(Cossins et al., 2002), suggesting that hes6.1 must be spatially regulated in the presomitic mesoderm for proper segmentation, perhaps by protein-protein interactions rather than
DNA binding. Interestingly, hes6.1 is negatively regulated by
Notch/RBPJ in the neural plate (Koyano-Nakagawa et al.,
2000), so it will be interesting to study a possible interplay
between hes6.1 and the Notch pathway in somitogenesis.
7.3.6.2. Interplay between Notch and Other
Genes and Pathways in Somitogenesis
Somite boundary formation is also regulated by the Notch
pathway via repression of protocadherin 8 (pcdh8) in the
posterior half of somitomeres. Pcdh8, which is expressed in
their anterior half, in turn regulates differential cell adhesion and prevents the intermingling of anterior and posterior cells between somitomeres, contributing to maintaining