FoxA4 Favours Notochord Formation by Inhibiting Contiguous Mesodermal Fates and Restricts Anterior Neural Development in Xenopus Embryos

Abstract

In vertebrates, the embryonic dorsal midline is a crucial signalling centre that patterns the surrounding tissues during development. Members of the FoxA subfamily of transcription factors are expressed in the structures that compose this centre. Foxa2 is essential for dorsal midline development in mammals, since knock-out mouse embryos lack a definitive node, notochord and floor plate. The related gene foxA4 is only present in amphibians. Expression begins in the blastula –chordin and –noggin expressing centre (BCNE) and is later restricted to the dorsal midline derivatives of the Spemann's organiser. It was suggested that the early functions of mammalian foxa2 are carried out by foxA4 in frogs, but functional experiments were needed to test this hypothesis. Here, we show that some important dorsal midline functions of mammalian foxa2 are exerted by foxA4 in Xenopus. We provide new evidence that the latter prevents the respecification of dorsal midline precursors towards contiguous fates, inhibiting prechordal and paraxial mesoderm development in favour of the notochord. In addition, we show that foxA4 is required for the correct regionalisation and maintenance of the central nervous system. FoxA4 participates in constraining the prospective rostral forebrain territory during neural specification and is necessary for the correct segregation of the most anterior ectodermal derivatives, such as the cement gland and the pituitary anlagen. Moreover, the early expression of foxA4 in the BCNE (which contains precursors of the whole forebrain and most of the midbrain and hindbrain) is directly required to restrict anterior neural development.

A gene regulatory program controlling early Xenopus mesendoderm formation: Network conservation and motifs

“…Finally, in Xenopus , foxa4 is the earliest and most abundantly expressed foxa gene during early mesendoderm specification. However, Foxa4's role in early mesendoderm development is not known, although by early neurula stages it promotes notochord formation and inhibits prechordal and paraxial mesoderm [132]. Our network analysis reveals that foxa4 is activated by Smad2/3, via a Foxh1-independent mechanism [26].…”

Segregation of brain and organizer precursors is differentially regulated by Nodal signaling at blastula stage

“…3 D); b) a posterior subdomain, corresponding to the notochordal cells (red arrow, Fig. 3 D), which ultimately form a rod by convergent-extension movements ( Murgan et al, 2014 ; Kwan and Kirschner, 2003 ). Cer-S- injected embryos showed that most chrd.1 expression consisted of the fan-shaped subdomain (green arrow, Fig.…”

“…The rostral forebrain is an evolutionary acquisition of vertebrates related to the appearance of the Hesx1 gene at the beginning of vertebrate evolution ( Ermakova et al, 1999 ; Wilson and Houart, 2004 ; Ermakova et al, 2007 ; Bayramov et al, 2016 ) and derives entirely from the BCNE center ( Kuroda et al, 2004 ). We have previously shown that expression of the rostral forebrain regulator hesx1 persists after blocking Nodal signaling in Xenopus ( Murgan et al, 2014 ). Indeed, the expansion of the Hesx1 domain in Nodal −/− mouse embryos occurred before gastrulation ( Camus et al, 2006 ).…”

“…Microinjection, culture, and fixation of embryos were performed as previously described ( Murgan et al, 2014 ). Cer-S mRNA was injected in the vegetal region of the four cells at s3 (0.5 ng/cell).…”

FoxD1 protein interacts with Wnt and BMP signaling to differentially pattern mesoderm and neural tissue

“…Whole-mount in-situ hybridization was performed with digoxigeninlabeled probes (Harland, 1991) to muscle actin (MA) and myod (Keren et al, 2005), chd (Murgan et al, 2014), and otx2, krox20 and hoxb9 (Aamar and Frank, 2004;Gutkovich et al, 2010).…”