**5. Importance of NFκB signaling in mediating early differentiation of ES/iPS**

*In vitro* neural induction from cultured ES cells or induced pluripotent stem (iPS) cells has been established [80, 82, 91, 92], but the signaling mechanisms remain largely unknown. Such induction is an excellent *in vitro* model to recapture the *in vivo* neural induction and embryonic neurogenesis [80]. The signaling pathways identified during endogenous embry‐ onic morphogenesis can be applied to the neural induction and patterning, such as BMP, FGF, Wnt, Shh and Notch signaling [77, 78, 79, 80, 81, 82]. We speculate that NFκB signaling, through crosstalk with these signaling pathways, play an important role in the neuronal in‐ duction from ES or iPS cells (Figure 5) [80].

During murine spermatogenesis, NFκB is activated in a stage-specific manner [93]. During oocyte maturation and early embryonic development, NFκB is activated [94, 95]. In Droso‐ phila melanogaster, the mRNA of the p65 homologue, named Dorsal, is maternally ex‐ pressed and is concentrated in the egg cortex [85]. In Xenopus, NFκB activation is observed during oocyte maturation [96] and in late blastulae and gastrulae [97]. In zebrafish, NFκB signaling regulates notochord differentiation via activating the expression of no tail (ntl) gene [98]. In mouse embryos, NFκB activation is crucial to engage development beyond the 2-cell stage [94]. NFκB mediates the neurogenic effect of erythropoietin in neurosphere cul‐ tures from E14 mouse ganglionic eminence [99]. Recently, it has been shown that murine and human ES cells possess a low level of NFκB activity that increases significantly during the differentiation process [100, 101, 102]. In human ES cells, the classical NFκB pathway regulates differentiation while the non-classical pathway maintains pluripotency [103]. The transcription factor Nanog is essential in maintaining pluripotency of ES cells [104]. During ES cell differentiation, endogenous NFκB activity and target-gene expression are increased (Figure 5) [101, 102, 105]. NFκB inhibition increases expression of pluripotency markers [106, 107]. Nanog binds to NFκB proteins, inhibits NFκB activity and cooperates with Stat3 to maintain pluripotency [100]. ES cell-specific miR-290 maintains the pluripotency and self-re‐ newal of ES cells through repressing classical NFκB signaling [107]. Forced expression of p65 causes loss of pluripotency, promotes differentiation of ES cells, and leads to an epithe‐ lial to mesenchymal transition [107]. These data define p65 as a novel target gene of miR-290 cluster and provide new insight into the function of ES cell-specific miRNAs [107]. Taken altogether, NFκB signaling is activated and required during the early differentiation of vari‐ ous stem cells and embryogenesis (Figure 5).

During neural induction, the ectodermal epithelial cells transit into NECs due to the inhibi‐ tion of bone morphogenetic protein (BMP) signaling by the neural inducer (Chordin, Nog‐ gin and Follistatin). In this original "default model", high activity of BMP signaling defines epidermis, while absence of BMP specifies neural plate [75, 76, 77]. However, this model can no longer explain the complicate process of neural induction, which involves additional sig‐ naling pathways such as Wnt/β-catenin, FGF, Sox2, and Notch signaling [77, 78, 79, 80, 81, 82]. NFκB signaling is shown to inhibit BMP signaling in oesteoblastogenesis [83, 84]. We speculate that NFκB may regulate neural induction. Previous studies showed that the grad‐ ed activation of NFκB/c-rel protein in the dorsal region determine the dorsal-ventral pat‐ terning in Drosophila [85, 86, 87, 88] and Xenopus laevis [89]. During mouse embryogenesis, virtually all members of the NFκB pathway are expressed in embryonic, trophoblast, and uterine cells [90]. It is proposed that NFκB may protect the embryos exposed to embryopath‐ ic stresses, possibly through its anti-apoptotic effect [90]. However, there is no direct evi‐

dence for the role of NFκB signaling in the *in vivo* neural induction (Figure 5).

**ES/iPS**

duction from ES or iPS cells (Figure 5) [80].

204 Trends in Cell Signaling Pathways in Neuronal Fate Decision

**5. Importance of NFκB signaling in mediating early differentiation of**

*In vitro* neural induction from cultured ES cells or induced pluripotent stem (iPS) cells has been established [80, 82, 91, 92], but the signaling mechanisms remain largely unknown. Such induction is an excellent *in vitro* model to recapture the *in vivo* neural induction and embryonic neurogenesis [80]. The signaling pathways identified during endogenous embry‐ onic morphogenesis can be applied to the neural induction and patterning, such as BMP, FGF, Wnt, Shh and Notch signaling [77, 78, 79, 80, 81, 82]. We speculate that NFκB signaling, through crosstalk with these signaling pathways, play an important role in the neuronal in‐

During murine spermatogenesis, NFκB is activated in a stage-specific manner [93]. During oocyte maturation and early embryonic development, NFκB is activated [94, 95]. In Droso‐ phila melanogaster, the mRNA of the p65 homologue, named Dorsal, is maternally ex‐ pressed and is concentrated in the egg cortex [85]. In Xenopus, NFκB activation is observed during oocyte maturation [96] and in late blastulae and gastrulae [97]. In zebrafish, NFκB signaling regulates notochord differentiation via activating the expression of no tail (ntl) gene [98]. In mouse embryos, NFκB activation is crucial to engage development beyond the 2-cell stage [94]. NFκB mediates the neurogenic effect of erythropoietin in neurosphere cul‐ tures from E14 mouse ganglionic eminence [99]. Recently, it has been shown that murine and human ES cells possess a low level of NFκB activity that increases significantly during the differentiation process [100, 101, 102]. In human ES cells, the classical NFκB pathway regulates differentiation while the non-classical pathway maintains pluripotency [103]. The transcription factor Nanog is essential in maintaining pluripotency of ES cells [104]. During ES cell differentiation, endogenous NFκB activity and target-gene expression are increased (Figure 5) [101, 102, 105]. NFκB inhibition increases expression of pluripotency markers [106, 107]. Nanog binds to NFκB proteins, inhibits NFκB activity and cooperates with Stat3 to

**Neural induction and neural fate decision**

**Figure 5.** Potential regulatory sites of NFκB signaling during neuronal cell fate decision. Solid green arrows indicate the sites supported by limited reports, while the dotted red arrows indicate the stages that need experimental sup‐ ports. ESC, embryonic stem cells; EB, embryoid body; NEC, neural epithelial cells; NSCs, neural stem cells; NPCs, neural progenitor cells.
