129
Evolution of the Vertebrate Neural Crest
to promote neural crest cell survival, which is antagonized by Msx2-mediated pro-apoptotic signals in the same
domain (Tríbulo et al. 2004). Although numerous targets
of Snai1 and Snai2 have been identifed at various stages
of neural crest ontogeny, much remains to be learned about
how snai1 and snai2 expression is regulated. There is evidence that BMP (Vallin et al. 2001), Wnt (Li, et al. 2019),
and retinoic acid signaling (Tamanoue et al. 2010), as well
as rho GTPases (Broders-Bondon et al. 2007), all activate
expression of snai1 and snai2. By contrast, YY1 appears
to repress snai2 expression at the lateral edge of the neural plate border, thereby ensuring that the domain of Snai2
activity does not encroach on the presumptive epidermal
ectoderm (Morgan et al. 2004).
The SRY-related HMG-box (sox) genes comprise a large
superfamily of transcription factors that play diverse roles
in metazoan embryogenesis. In vertebrates, the SoxE (Sox8,
Sox9, Sox10) and SoxD (Sox5, Sox6) subfamilies play reiterated roles in neural crest development. In Xenopus, sox8,
sox9, and sox10 are expressed sequentially (BuitragoDelgado et al. 2018) and direct neural crest stem cell formation (Spokony et al. 2002; Honoré et al. 2003; Lee et al.
2004; O’Donnell et al. 2006). Similarly, sox5 is expressed in
premigratory neural crest and morpholino-mediated knockdown leads to loss of neural crest gene expression (Nordin
and LaBonne 2014).
Other factors involved in establishing neural crest stem
cells include Tfap2 (alpha, beta and gamma, epsilon variants)
(Gotoh et al. 2003; Zhang et al. 2006; de Crozé et al. 2011;
Hong et al. 2014), Myc (Bellmeyer et al. 2003), Id3 (Light et al.
2005), and Hes4/Hairy2 (Nagatomo and Hashimoto 2007).
These proteins are all required for neural crest formation and
maintenance of pluripotency. id3 is a key Myc transcriptional
target in neural crest stem cells (Reynaud-Deonauth et al.
2002; Light et al. 2005) and controls lineage restriction in
these cells by regulating the balance between proliferation
and cell death (Kee and Bronner-Fraser 2005). In Xenopus
neural crest, Id3 can be regulated in part by physical interactions with Hes4/Hairy2 (Nichane et al. 2008). Moreover,
both Hes4/Hairy2 and Id3 regulate, and are regulated by,
Stat3.1 signaling, with high levels of stat3.1 promoting pluripotent maintenance and low levels triggering differentiation
(Nichane et al. 2010). The transcription factor Ets1 also drives
neural crest specifcation and migration via interactions with
Vgll3 (Simon et al. 2017) and promotes formation of cardiac
neural crest in Xenopus (Nie and Bronner 2015).
More work is needed to identify the direct transcriptional
targets of each of the key neural crest factors to fully understand their individual regulatory contributions, and Xenopus
provides an ideal system for such studies. Importantly, a number of these factors can act as both transcriptional activators
and repressors in a context-dependent manner, complicating
efforts to build simple GRNs depicting their regulatory hierarchies. In addition, the activities of a number of neural crest
regulatory factors have been shown to be regulated by posttranslational modifcations, including phosphorylation and
SUMOylation (Taylor and LaBonne 2007). Studies in Xenopus
have shown that although Sox9 and Sox10 can promote the
neural crest stem cell state as well as glia and melanocyte
formation, their activity can be modifed by SUMOylation,
which converts them to transcriptional repressors and promotes ear formation (Taylor and LaBonne 2005; Lee, TaylorJaffe et al. 2012). Similarly, the activity of Twist1 and Snai1
in the neural crest is regulated by ubiquitination (Vernon and
LaBonne 2006; Lander et al. 2011, 2013). Understanding both
post-translational regulation and transcriptional targets is thus
essential to understanding how individual neural crest factors
contribute to the control of developmental potential, the control of invasive and migratory behavior, and the capacity to
give rise to specifc lineage states.
8.3.2. NEURAL CREST EMT AND MIGRATION
The establishment of a defnitive neural crest state requires
gene regulatory circuits to maintain the broad developmental potential of these cells while endowing them with the
capacity for migratory and invasive behavior and potentially
biasing them in their capacity to transit to specif c lineage
fates. The set of identifed GRN components functioning at these stages in Xenopus is quite large (Pegoraro and
Monsoro-Burq 2013), and there is likewise substantial variation in gene regulatory activity across the vertebrate phylogeny. However, a common suite of proteins and signaling
pathways for enabling EMT and migration are deployed by
representatives of all major vertebrate lineages studied to
date. These include Twist1, Snai1/Snai2, Tfap2a/b, Foxd3,
Ets1, Myc, Id3 and signaling pathways including Endothelin,
BMP, Wnt, and FGF, as well as novel factors such as Anos1,
Phb, Slc19a1/rfc, Zfand6/awp1, and Sp5 (Table 8.1).
The proteins Twist1 and Twist2 are bHLH transcription factors that play key roles in mesodermal development
TABLE 8.1
Novel/Non-Canonical Genes Identified as Being Essential
for Regulating the Neural Crest GRN
Gene
Functional Roles
References
kctd15
Inhibits neural crest formation
Dutta and Dawid 2010
apoc1
Induction/Wnt signaling
Tamai et al. 2000
hes3
Induction and NPB
Hong and Saint-Jeannet
establishment
2018
dkk2
NPB establishment/Wnt
Devotta et al. 2018
signaling
adam33
Induction/Wnt signaling
Wei et al. 2010
adam19
Induction/Wnt signaling
Li et al. 2018
szl
Induction/Wnt signaling
Salic et al. 1997
anos1
Formation of cranial neural
Bae et al. 2018
crest/placodes
zfand6/
Early neural crest formation
Seo et al. 2014
awp1
phb
Early neural crest formation
Schneider et al. 2010 ;
Deichmann et al. 2015
sp5
Early neural crest formation
Park et al. 2013
slc19a1/rfc
Epigenetic regulation
Li et al. 2011
Evolution of the Vertebrate Neural Crest
to promote neural crest cell survival, which is antagonized by Msx2-mediated pro-apoptotic signals in the same
domain (Tríbulo et al. 2004). Although numerous targets
of Snai1 and Snai2 have been identifed at various stages
of neural crest ontogeny, much remains to be learned about
how snai1 and snai2 expression is regulated. There is evidence that BMP (Vallin et al. 2001), Wnt (Li, et al. 2019),
and retinoic acid signaling (Tamanoue et al. 2010), as well
as rho GTPases (Broders-Bondon et al. 2007), all activate
expression of snai1 and snai2. By contrast, YY1 appears
to repress snai2 expression at the lateral edge of the neural plate border, thereby ensuring that the domain of Snai2
activity does not encroach on the presumptive epidermal
ectoderm (Morgan et al. 2004).
The SRY-related HMG-box (sox) genes comprise a large
superfamily of transcription factors that play diverse roles
in metazoan embryogenesis. In vertebrates, the SoxE (Sox8,
Sox9, Sox10) and SoxD (Sox5, Sox6) subfamilies play reiterated roles in neural crest development. In Xenopus, sox8,
sox9, and sox10 are expressed sequentially (BuitragoDelgado et al. 2018) and direct neural crest stem cell formation (Spokony et al. 2002; Honoré et al. 2003; Lee et al.
2004; O’Donnell et al. 2006). Similarly, sox5 is expressed in
premigratory neural crest and morpholino-mediated knockdown leads to loss of neural crest gene expression (Nordin
and LaBonne 2014).
Other factors involved in establishing neural crest stem
cells include Tfap2 (alpha, beta and gamma, epsilon variants)
(Gotoh et al. 2003; Zhang et al. 2006; de Crozé et al. 2011;
Hong et al. 2014), Myc (Bellmeyer et al. 2003), Id3 (Light et al.
2005), and Hes4/Hairy2 (Nagatomo and Hashimoto 2007).
These proteins are all required for neural crest formation and
maintenance of pluripotency. id3 is a key Myc transcriptional
target in neural crest stem cells (Reynaud-Deonauth et al.
2002; Light et al. 2005) and controls lineage restriction in
these cells by regulating the balance between proliferation
and cell death (Kee and Bronner-Fraser 2005). In Xenopus
neural crest, Id3 can be regulated in part by physical interactions with Hes4/Hairy2 (Nichane et al. 2008). Moreover,
both Hes4/Hairy2 and Id3 regulate, and are regulated by,
Stat3.1 signaling, with high levels of stat3.1 promoting pluripotent maintenance and low levels triggering differentiation
(Nichane et al. 2010). The transcription factor Ets1 also drives
neural crest specifcation and migration via interactions with
Vgll3 (Simon et al. 2017) and promotes formation of cardiac
neural crest in Xenopus (Nie and Bronner 2015).
More work is needed to identify the direct transcriptional
targets of each of the key neural crest factors to fully understand their individual regulatory contributions, and Xenopus
provides an ideal system for such studies. Importantly, a number of these factors can act as both transcriptional activators
and repressors in a context-dependent manner, complicating
efforts to build simple GRNs depicting their regulatory hierarchies. In addition, the activities of a number of neural crest
regulatory factors have been shown to be regulated by posttranslational modifcations, including phosphorylation and
SUMOylation (Taylor and LaBonne 2007). Studies in Xenopus
have shown that although Sox9 and Sox10 can promote the
neural crest stem cell state as well as glia and melanocyte
formation, their activity can be modifed by SUMOylation,
which converts them to transcriptional repressors and promotes ear formation (Taylor and LaBonne 2005; Lee, TaylorJaffe et al. 2012). Similarly, the activity of Twist1 and Snai1
in the neural crest is regulated by ubiquitination (Vernon and
LaBonne 2006; Lander et al. 2011, 2013). Understanding both
post-translational regulation and transcriptional targets is thus
essential to understanding how individual neural crest factors
contribute to the control of developmental potential, the control of invasive and migratory behavior, and the capacity to
give rise to specifc lineage states.
8.3.2. NEURAL CREST EMT AND MIGRATION
The establishment of a defnitive neural crest state requires
gene regulatory circuits to maintain the broad developmental potential of these cells while endowing them with the
capacity for migratory and invasive behavior and potentially
biasing them in their capacity to transit to specif c lineage
fates. The set of identifed GRN components functioning at these stages in Xenopus is quite large (Pegoraro and
Monsoro-Burq 2013), and there is likewise substantial variation in gene regulatory activity across the vertebrate phylogeny. However, a common suite of proteins and signaling
pathways for enabling EMT and migration are deployed by
representatives of all major vertebrate lineages studied to
date. These include Twist1, Snai1/Snai2, Tfap2a/b, Foxd3,
Ets1, Myc, Id3 and signaling pathways including Endothelin,
BMP, Wnt, and FGF, as well as novel factors such as Anos1,
Phb, Slc19a1/rfc, Zfand6/awp1, and Sp5 (Table 8.1).
The proteins Twist1 and Twist2 are bHLH transcription factors that play key roles in mesodermal development
TABLE 8.1
Novel/Non-Canonical Genes Identified as Being Essential
for Regulating the Neural Crest GRN
Gene
Functional Roles
References
kctd15
Inhibits neural crest formation
Dutta and Dawid 2010
apoc1
Induction/Wnt signaling
Tamai et al. 2000
hes3
Induction and NPB
Hong and Saint-Jeannet
establishment
2018
dkk2
NPB establishment/Wnt
Devotta et al. 2018
signaling
adam33
Induction/Wnt signaling
Wei et al. 2010
adam19
Induction/Wnt signaling
Li et al. 2018
szl
Induction/Wnt signaling
Salic et al. 1997
anos1
Formation of cranial neural
Bae et al. 2018
crest/placodes
zfand6/
Early neural crest formation
Seo et al. 2014
awp1
phb
Early neural crest formation
Schneider et al. 2010 ;
Deichmann et al. 2015
sp5
Early neural crest formation
Park et al. 2013
slc19a1/rfc
Epigenetic regulation
Li et al. 2011