99
Notch Signaling in Early Embryogenesis
(Pichon et al., 2002; Shook et al., 2004) (Figure 7.2). Since
hey1 is also expressed in scattered cells in the involuted dorsal IMZ at early gastrula (see clone XL097h17 in Taverner
et al., 2005), it will be interesting to study if it promotes
hypochordal fates over notochord. hey1 is also expressed
in the FP during neurulation, but neither time-controlled
hey1 overexpression at late gastrula nor hey1 knock-down
affected FP development when analyzed at tailbud stages
(Taelman et al., 2004). There is evidence that hey1 might
suppress neurogenesis in the FP by antagonizing proneural
genes, thereby maintaining FP identity. Interestingly, neurog2 overexpression revealed FP’s potential to differentiate
into neurons, and hey1 overexpression suppressed primary
neurogenesis in the neural plate. However, neither hey1
knock-down alone nor combined with hes4.L knock-down
induced ectopic neurogenesis in the FP, indicating that additional inhibitors might be required to inhibit neurogenesis
(Taelman et al., 2004).
7.3.4. PRIMARY NEUROGENESIS
Differentiation of multipotent neural progenitors into the
diverse nervous system cell types is an orchestrated process
ensuring that neurons and glia appear at the right time and
place during development. Key players in this process are
Notch pathway components and several bHLH proteins.
Those encoded by “proneural” genes—members of the
Neurogenin and Achaete-scute families—heterodimerize
with the bHLH factor E47, bind the E box (CANNTG), and
activate transcription, promoting competence for neuronal
differentiation. Downstream, other bHLH transcriptional
activators promote the determination of neurons (NeuroD
family) or oligodendrocytes (Olig family). Notch-regulated
Hes proteins typically repress proneural gene expression or
activity, maintaining neural precursors in a proliferative and
undifferentiated state, and allow astrocyte differentiation
(Davis and Turner, 2001; Bertrand et al., 2002; Huang et
al., 2014; Kageyama et al., 2007; Imayoshi and Kageyama,
2014 ).
Anamniotes develop through a larval period that requires
a simple neuronal circuitry for swimming and escape
refexes to be functioning around hatching (Roberts, 1989).
In Xenopus, a frst wave of primary neurogenesis, which
begins at late gastrula and peaks at neural plate stages, generates three bilateral pairs of longitudinal stripes of “primary
neurons”: motoneurons, interneurons, and sensory neurons
that are responsible for these larval behaviors (Chitnis et
al., 1995). Study of primary neurogenesis in Xenopus signif cantly contributed to the discovery of the molecular and
cellular basis of vertebrate neurogenesis and provided an
accessible paradigm to study the Notch pathway (Tables 7.7,
7.8). The neurogenesis gene regulatory network (GRN) built
from this work is initially controlled by the balanced expression of “prepattern genes,” such as those encoding Gli and
Zic TFs (Lee et al., 1997; Marine et al., 1997; Brewster et
al., 1998; Nakata et al., 1998). By refning proneural gene
expression, the prepattern TFs roughly outline regions in the
neural plate in which primary neuronal differentiation can
or cannot occur; this confers neuronal differentiation competence to restricted domains ( Zimmerman et al., 1993; Ma
et al., 1996 ).
Proneural genes induce notch1 and dll1, whose expression in the posterior neural plate begins around late gastrula in overlapping stripes that prefgure the placement of
the primary neurons (Turner and Weintraub, 1994; Chitnis
et al., 1995; Ma et al., 1996; Chitnis and Kintner, 1996 )
( Figure 7.1C ). notch1 is expressed by most cells in the proneural domain, whereas dll1 is restricted to a subset of them
(Chitnis et al., 1995; Ma et al., 1996; Chalmers et al., 2002).
Primary neurogenesis is circumscribed by Notch-dependent
lateral inhibition: the selected neuronal precursor expresses
Dll1, which binds Notch1 in neighboring cells, activating the
Notch/Psen/RBPJ/Maml pathway, resulting in the induction
of Hes1–7 bHLH-O repressors that inhibit proneural genes
and thereby repress neuronal fate in the neighbors (Chitnis
et al., 1995; Ma et al., 1996; Bellefroid et al., 1996; Wettstein
et al., 1997; Perron et al., 1999; Paganelli et al., 2001; Katada
and Kinoshita, 2003; López et al., 2003; Nichane et al.,
2008b; Revinski et al., 2010; Riddiford and Schlosser, 2017)
(Tables 7.3, 7.4, 7.8). A negative feedback loop is established
in the neuronal precursors that suppresses dll1 expression
in their neighbors through NICD1. The neuronal precursors continue to express proneural genes, which induce the
bHLH-determination factor neurod1. Once neurod1 is activated, the cells become refractory to lateral inhibition and
undergo terminal differentiation into neurons (Chitnis et al.,
1995; Chitnis and Kintner, 1996; Olson et al., 1998; Sjöqvist
and Andersson, 2019). Another pathway for preventing lateral inhibition is through the upregulation of the zinc-f nger
TF myt1 by Neurog2 (Bellefroid et al., 1996). In other scenarios, when proneural TFs reach a certain threshold, they
induce Ebf2 in selected progenitors, which stabilizes commitment to a neuronal fate by enhancing dll1 expression
and reinforcing neurod1 expression (Dubois et al., 1998).
neurod1 appears to feed back to directly potentiate dll1
expression, as it promotes ectopic dll1 in whole embryos and
induces dll1 in animal caps in the absence of protein synthesis (Seo et al., 2007).
7.3.4.1. Notch Ligands
In the neural plate, dll1 expression is stronger posteriorly, and jag1 expression is stronger anteriorly (Table 7.7)
(Figure 7.1C) (Kiyota et al., 2001). Jag1 normally restricts
the differentiation of primary neurons, and combined dll1
and jag1 expression is indispensable for the normal primary neurogenesis pattern (Kiyota et al., 2001) (Table 7.7 ).
Interestingly, Dll1 and Jag1 contain sequences encoding a
putative nuclear localization signal. Moreover, GFP fusion
proteins of both ligands naturally underwent proteolitic
cleavage during gastrulation, releasing their intracellular
domains (ICDs), which were detected in nuclei. However,
only Jag1-ICD-GFP persisted in cell nuclei and repressed
primary neurogenesis without activating hes5.1 ( Kiyota and
Kinoshita, 2004). It was proposed that Jag1 inhibits primary
Notch Signaling in Early Embryogenesis
(Pichon et al., 2002; Shook et al., 2004) (Figure 7.2). Since
hey1 is also expressed in scattered cells in the involuted dorsal IMZ at early gastrula (see clone XL097h17 in Taverner
et al., 2005), it will be interesting to study if it promotes
hypochordal fates over notochord. hey1 is also expressed
in the FP during neurulation, but neither time-controlled
hey1 overexpression at late gastrula nor hey1 knock-down
affected FP development when analyzed at tailbud stages
(Taelman et al., 2004). There is evidence that hey1 might
suppress neurogenesis in the FP by antagonizing proneural
genes, thereby maintaining FP identity. Interestingly, neurog2 overexpression revealed FP’s potential to differentiate
into neurons, and hey1 overexpression suppressed primary
neurogenesis in the neural plate. However, neither hey1
knock-down alone nor combined with hes4.L knock-down
induced ectopic neurogenesis in the FP, indicating that additional inhibitors might be required to inhibit neurogenesis
(Taelman et al., 2004).
7.3.4. PRIMARY NEUROGENESIS
Differentiation of multipotent neural progenitors into the
diverse nervous system cell types is an orchestrated process
ensuring that neurons and glia appear at the right time and
place during development. Key players in this process are
Notch pathway components and several bHLH proteins.
Those encoded by “proneural” genes—members of the
Neurogenin and Achaete-scute families—heterodimerize
with the bHLH factor E47, bind the E box (CANNTG), and
activate transcription, promoting competence for neuronal
differentiation. Downstream, other bHLH transcriptional
activators promote the determination of neurons (NeuroD
family) or oligodendrocytes (Olig family). Notch-regulated
Hes proteins typically repress proneural gene expression or
activity, maintaining neural precursors in a proliferative and
undifferentiated state, and allow astrocyte differentiation
(Davis and Turner, 2001; Bertrand et al., 2002; Huang et
al., 2014; Kageyama et al., 2007; Imayoshi and Kageyama,
2014 ).
Anamniotes develop through a larval period that requires
a simple neuronal circuitry for swimming and escape
refexes to be functioning around hatching (Roberts, 1989).
In Xenopus, a frst wave of primary neurogenesis, which
begins at late gastrula and peaks at neural plate stages, generates three bilateral pairs of longitudinal stripes of “primary
neurons”: motoneurons, interneurons, and sensory neurons
that are responsible for these larval behaviors (Chitnis et
al., 1995). Study of primary neurogenesis in Xenopus signif cantly contributed to the discovery of the molecular and
cellular basis of vertebrate neurogenesis and provided an
accessible paradigm to study the Notch pathway (Tables 7.7,
7.8). The neurogenesis gene regulatory network (GRN) built
from this work is initially controlled by the balanced expression of “prepattern genes,” such as those encoding Gli and
Zic TFs (Lee et al., 1997; Marine et al., 1997; Brewster et
al., 1998; Nakata et al., 1998). By refning proneural gene
expression, the prepattern TFs roughly outline regions in the
neural plate in which primary neuronal differentiation can
or cannot occur; this confers neuronal differentiation competence to restricted domains ( Zimmerman et al., 1993; Ma
et al., 1996 ).
Proneural genes induce notch1 and dll1, whose expression in the posterior neural plate begins around late gastrula in overlapping stripes that prefgure the placement of
the primary neurons (Turner and Weintraub, 1994; Chitnis
et al., 1995; Ma et al., 1996; Chitnis and Kintner, 1996 )
( Figure 7.1C ). notch1 is expressed by most cells in the proneural domain, whereas dll1 is restricted to a subset of them
(Chitnis et al., 1995; Ma et al., 1996; Chalmers et al., 2002).
Primary neurogenesis is circumscribed by Notch-dependent
lateral inhibition: the selected neuronal precursor expresses
Dll1, which binds Notch1 in neighboring cells, activating the
Notch/Psen/RBPJ/Maml pathway, resulting in the induction
of Hes1–7 bHLH-O repressors that inhibit proneural genes
and thereby repress neuronal fate in the neighbors (Chitnis
et al., 1995; Ma et al., 1996; Bellefroid et al., 1996; Wettstein
et al., 1997; Perron et al., 1999; Paganelli et al., 2001; Katada
and Kinoshita, 2003; López et al., 2003; Nichane et al.,
2008b; Revinski et al., 2010; Riddiford and Schlosser, 2017)
(Tables 7.3, 7.4, 7.8). A negative feedback loop is established
in the neuronal precursors that suppresses dll1 expression
in their neighbors through NICD1. The neuronal precursors continue to express proneural genes, which induce the
bHLH-determination factor neurod1. Once neurod1 is activated, the cells become refractory to lateral inhibition and
undergo terminal differentiation into neurons (Chitnis et al.,
1995; Chitnis and Kintner, 1996; Olson et al., 1998; Sjöqvist
and Andersson, 2019). Another pathway for preventing lateral inhibition is through the upregulation of the zinc-f nger
TF myt1 by Neurog2 (Bellefroid et al., 1996). In other scenarios, when proneural TFs reach a certain threshold, they
induce Ebf2 in selected progenitors, which stabilizes commitment to a neuronal fate by enhancing dll1 expression
and reinforcing neurod1 expression (Dubois et al., 1998).
neurod1 appears to feed back to directly potentiate dll1
expression, as it promotes ectopic dll1 in whole embryos and
induces dll1 in animal caps in the absence of protein synthesis (Seo et al., 2007).
7.3.4.1. Notch Ligands
In the neural plate, dll1 expression is stronger posteriorly, and jag1 expression is stronger anteriorly (Table 7.7)
(Figure 7.1C) (Kiyota et al., 2001). Jag1 normally restricts
the differentiation of primary neurons, and combined dll1
and jag1 expression is indispensable for the normal primary neurogenesis pattern (Kiyota et al., 2001) (Table 7.7 ).
Interestingly, Dll1 and Jag1 contain sequences encoding a
putative nuclear localization signal. Moreover, GFP fusion
proteins of both ligands naturally underwent proteolitic
cleavage during gastrulation, releasing their intracellular
domains (ICDs), which were detected in nuclei. However,
only Jag1-ICD-GFP persisted in cell nuclei and repressed
primary neurogenesis without activating hes5.1 ( Kiyota and
Kinoshita, 2004). It was proposed that Jag1 inhibits primary