Msx1 and Pax3 Cooperate to Mediate FGF8 and WNT Signals during Xenopus Neural Crest Induction

Abstract

FGF, WNT, and BMP signaling promote neural crest formation at the neural plate boundary in vertebrate embryos. To understand how these signals are integrated, we have analyzed the role of the transcription factors Msx1 and Pax3. Using a combination of overexpression and morpholino-mediated knockdown strategies in Xenopus, we show that Msx1 and Pax3 are both required for neural crest formation, display overlapping but nonidentical activities, and that Pax3 acts downstream of Msx1. In neuralized ectoderm, Msx1 is sufficient to induce multiple early neural crest genes. Msx1 induces Pax3 and ZicR1 cell autonomously, in turn, Pax3 combined with ZicR1 activates Slug in a WNT-dependent manner. Upstream of this, WNTs initiate Slug induction through Pax3 activity, whereas FGF8 induces neural crest through both Msx1 and Pax3 activities. Thus, WNT and FGF8 signals act in parallel at the neural border and converge on Pax3 activity during neural crest induction.

AP2γ regulates neural and epidermal development downstream of the BMP pathway at early stages of ectodermal patterning

“…Furthermore, AP2γ has redundant activities with AP2α in neural crest induction and nonneural ectoderm derivative development [26]. Other factors such as Pax7, Msx1, Zic1, Dlx5 and AP2α were shown as neural border specifiers [30,[63][64][65][66][67][68]. During early neuralization of the chick embryo, some factors are mainly expressed in the peripheral (Pax7, Dlx5 and AP2α) or caudal (Msx1) ectoderm and the expressions of others (Zic1) partially overlaps with cSox2 expression [68].…”

Xenopus hairy2 functions in neural crest formation by maintaining cells in a mitotic and undifferentiated state

“…Several recent reports have indicated that Wnts are neural crest inducers (reviewed by Raible, 2006). It was reported that the cooperative function of Pax3 and Zic1, in concert with Wnt signaling, is essential for neural crest specification (Monsoro-Burq et al, 2005;Sato et al, 2005). The transcription factor Xsnail, which is expressed as early as Xhairy2 in the prospective neural crest, is known to have the ability to induce neural crest marker genes (Aybar et al, 2003).…”

Signaling pathways and tissue interactions in neural plate border formation

“…17 In chick as well as in Xenopus and zebrafish, NPB markers are expressed at mid-to late gastrula stages. 10,18,19 A sharp and defined NPB is observed in chick embryos during and after neurulation, 10,20 and it appears that the competence to form NC is retained longer, well into neurula stages, in chick ectoderm as compared to the frog. 7,21,22 This discrepancy is likely due to the timing of gastrulation and neurulation in amphibian and chick embryos.…”

“…30,39 BMP and Wnt/b-Catenin signaling functionally interact with Pax3, Zic1 and Ap2a in later phases to induce bona fide NC fate. 19,30,40,41 Likewise, Wnt/b-Catenin signaling is essential for NPB specification in epiblast explants from chick blastulae, where Wnt activity precedes and upregulates bmp4, 17 as well as for expression of NPB genes in hESCs. 11 Steventon and colleagues have analyzed Wnt/ b-Catenin activity in Xenopus embryos using the TOPFlash reporter gene assay.…”

“…FGFs have been shown to sensitize the dorsal ectoderm to respond to BMP inhibition, transcriptionally regulate BMPs and Wnts, and to contribute directly to NPB specification. 19,20,[54][55][56] In Xenopus, the DLMZ expresses a number of paracrine growth factors at the onset of gastrulation including wnt8, fgf4, fgf8, gdf6 and the BMP antagonist chordin (Fig. 2) and further expresses hairy2, fgfr1, fgfr4a, bmpr1a, pcns and ror2.…”