br Conclusion br Acknowledgement This work was supported by

Conclusion
Acknowledgement This work was supported by JSPS KAKENHI (Grant Number 15K18770).
Introduction Neural crest AXL1717 are multipotent progenitors in the vertebrate embryo that give rise to a vast array of different cell types including pigment cells, craniofacial skeleton and mesenchyme, peripheral neurons and glia, cardiac and adrenal medulla derivatives (Bronner and LeDouarin, 2012, Kirby and Hutson, 2010). The presumptive neural crest (NC) is induced during gastrulation and neurulation, within a broad frontier area located across the neural plate and the nonneural ectoderm, named the neural (plate) border (NB) (see review by Pla and Monsoro-Burq, 2018 this issue). This domain is a mixed territory, which also contributes to the dorsal neural tube, the nonneural ectoderm and, anteriorly, to cranial placode progenitors (Steventon et al., 2009, Streit, 2002; reviewed in Pegoraro and Monsoro-Burq, 2013). The NB is induced by a combination of signals secreted from the adjacent tissues, namely the neural and non-neural dorsal ectoderm and the underlying paraxial and intermediate mesoderm. FGF signals, diffusing from the mesoderm, and Wnt ligands from both non-neural ectoderm and mesoderm, cooperate with low-level BMP signaling to activate the expression of the neural border specifiers pax3/7, zic1/2, tfap2a, hes4, or msx1/2, which are transcription factors required to establish the NB territory and are necessary for NC development (Basch and Bronner-Fraser, 2006, de Crozé et al., 2011, Mizuseki et al., 1998, Monsoro-Burq et al., 2005, Nichane et al., 2008a, Sasai, 2005, Tribulo, 2003). In turn, these factors synergize with WNT signals to activate the NC developmental program (Monsoro-Burq et al., 2005, Sasai, 2005, Simões-Costa et al., 2015). Further NC development follows successive steps: induction, epithelial-to-mesenchymal transition (EMT), migration and differentiation. This complex process is controlled by a network of transcription factors and soluble molecules: the NC gene regulatory network (NC-GRN), which has been validated in vivo, in multiple animal models including frog, chick and fish for the early stages of the network (Garnett et al., 2012, Milet and Monsoro-Burq, 2012, Sauka-Spengler and Bronner-Fraser, 2008). Multiple transcription factors are expressed and cooperate at each step of the network; for example, snai2, foxd3, sox8/9/10, twist1, ets1 are expressed in premigratory NC and are essential for NC specification and EMT. As our knowledge of the NC-GRN deepens, the main phases of NC formation can be subdivided into more precise sub-steps. To illustrate, during premigratory NC specification, two phases can be distinguished: first, the early premigratory NC cells (NCC), marked by expression of the transcription factors snai2, sox8 and foxd3, are induced and the immature NC population is maintained and amplified by action of BMP, Wnt and Notch signaling (Faure et al., 2002, Garcia-Castro et al., 2002, Nichane et al., 2008a, Nichane et al., 2008b); then, at neural fold stage, mature premigratory NCC express later NC specifiers, such as sox10 and twist, and activate numerous cell modifications that are essential for NCC EMT and migration. Until recently, very little evidence had implicated AKT signaling in NC early development. However, several new studies in fish, frog and mouse embryos have shown an essential role for AKT at each step of NC formation, from induction to differentiation. The serine/threonine kinase AKT, also called PKB for protein kinase B, is a major node in cell signaling for cell homeostasis. AKT protein comprises 4 main domains: a N-terminal pleckstrin homology (PH) domain, which recognizes and binds phosphatidylinositol-3,4,5-trisphosphate (PIP3) and allows AKT recruitment at the plasma membrane; a linker domain, a kinase domain and a C-terminal regulatory domain (Fig. 1A; Manning and Toker, 2017; Ruan and Kazlauskas, 2011). AKT acts downstream of signaling events recruiting PI3-kinase (PI3K) at the plasma membrane, such as the activation of receptor tyrosine kinases (e.g. PDGFRs, FGFRs, EGFRs, insulin receptors), of integrins, of B and T cell receptors, of cytokine receptors, or of G-protein-coupled receptors (Hers et al., 2011, Manning and Cantley, 2007, Manning and Toker, 2017). PI3K phosphorylates phosphatidyl-inositol-4,5-bisphosphate (PIP2) into PIP3, while the phosphatase PTEN catalyzes the reverse reaction (Divecha and Irvine, 1995, Maehama and Dixon, 1998). The accumulation of PIP3 at the plasma membrane simultaneously recruits AKT and PDK1 (3-phosphoinositide-dependent protein kinase 1), via their respective PH-domains proteins (Fig. 1B). AKT is then activated by two sequential phosphorylation events. First, PDK1 phosphorylates AKT on Thr-308, which stabilizes the activation loop located in the kinase domain. Secondly, the complex mTORC2 phosphorylates AKT on Ser-473, located in the C-terminal regulatory domain (Alessi et al., 1997, Alessi et al., 1996, Sarbassov et al., 2005, Stokoe et al., 1997). These two phosphorylation events are essential to attain maximal AKT activation. Moreover, many other post-translational modifications, including additional phosphorylations on Ser/Thr/Tyr residues, acetylation/ubiquitylation/methylation on lysine residues, hydroxylation, glycosylation, and SUMOylation participate in the fine tuning of AKT stability, activity, sub-cellular localization and partner selection (Risso et al., 2015). In turn, AKT controls multiple downstream effector pathways to monitor various major cellular functions. Among over a hundred different substrates reported by in vitro or in vivo experiments, active AKT controls the activity of FoxO proteins, regulators of cell survival, proliferation, growth and metabolism; of GSK3 which has major roles in survival, proliferation, metabolism and neural development; of mTORC1 known to act in protein synthesis and growth signaling; of p53 controlling cell cycle but also cell survival migration and metabolism; and of YAP broadly involved in proliferation, survival, growth and differentiation (Ma et al., 2007, Manning and Cantley, 2007, Moroishi et al., 2015, Piccolo et al., 2014, Roger et al., 2006, Tokino and Nakamura, 2000; Fig. 1B). Several of these targets have been studied in NC development, albeit not specifically as effectors of AKT signaling. GSK3 is a negative regulator of the Wnt-βcatenin signaling pathway, essential for NC development (see below). In frog embryos, FoxO4 depletion leads to craniofacial and pigmentation defects (Schuff et al., 2010). Defective p53 expression causes craniofacial defect in fish, mouse and chick (Rinon et al., 2011, Xia et al., 2013). Finally, YAP is important for mouse craniofacial development (Ma et al., 2007, Manning and Cantley, 2007, Moroishi et al., 2015, Piccolo et al., 2014, Roger et al., 2006, Tokino and Nakamura, 2000, Wang et al., 2016) as well as for NC stemness in human NCC in vitro (Hindley et al., 2016).