Patterning the forebrain: FoxA4a/Pintallavis and Xvent2 determine the posterior limit ofXanf1expression in the neural plate

Abstract

During early development of the nervous system in vertebrates, expression of the homeobox gene Anf/Hesx1/Rpx is restricted to the anterior neural plate subdomain corresponding to the presumptive forebrain. This expression is essential for normal forebrain development and ectopic expression of Xenopus Anf, Xanf1 (also known as Xanf-1),results in severe forebrain abnormalities. By use of transgenic embryos and a novel bi-colour reporter technique, we have identified a cis-regulatory element responsible for transcriptional repression of Xanf1 that defines its posterior expression limit within the neural plate. Using this element as the target in a yeast one-hybrid system, we identified two transcription factors, FoxA4a/Pintallavis and Xvent2 (also known as Xvent-2),which are normally expressed posterior to Xanf1. Overexpression of normal and dominant-negative versions of these factors, as well as inhibition of their mRNA translation by antisense morpholinos, show that they actually function as transcriptional repressors of Xanf1 just behind its posterior expression limit. The extremely high similarity of the identified Anf cis-regulatory sequences in Xenopus, chick and human,indicates that the mechanism restricting posterior expression of Anfin Xenopus is shared among vertebrates. Our findings support Nieuwkoop's activation-transformation model for neural patterning, according to which the entire neurectoderm is initially specified towards an anterior fate, which is later suppressed posteriorly as part of the trunk formation process.

Ras-dva, a member of novel family of small GTPases, is required for the anterior ectoderm patterning in theXenopus laevisembryo

“…The XlFgf8a cDNA was obtained by PCR with the following primers: forward 5Ј-AGAATTC-CCACCATGAACTACATCACCTCC; reverse 5Ј-ACTCGAGTTACCGA-GAACTTGAATATC. The VP16-Xanf1-BDG, EnR-Xanf1-BDGR, Otx2, Otx2-BDGR and Noggin constructs have been described previously (Ermakova et al, 1999;Martynova et al, 2004;Lamb et al, 1993). Capped mRNAs for microinjections were synthesized by mMESSAGE mMACHINE kit (Ambion).…”

Of Fox and Frogs: Fox (fork head/winged helix) transcription factors in Xenopus development

“…By ectopic expression it was found to induce the floor plate marker F-spondin (Ruiz i Altaba et al, 1993a). Shortly after, FoxA4 was shown to synergistically induce mesodermal derivatives in combination with Xbra (O'Reilly et al, 1995) and, as already mentioned, FoxA4 was recently shown to be involved in the patterning of the forebrain (Martynova et al, 2004). Interestingly, for both FoxA2 and FoxA4, an interplay with homeodomain proteins has been described.…”

“…Besides Mix.2, which was shown to be directly regulated by FoxH1a and H1b (Chen et al, 1996;Howell et al, 2002), FoxA4a was recently found to bind directly and to repress together with Xvent-2 the Xanf1 promoter, thus restricting the expression area of Xanf1 to the anterior neural plate (Martynova et al, 2004). This is of special interest because up to now, all members of the FoxA subclass were thought to function as activators (e.g., reviewed in Kaestner, 2000).…”

Expression of the transcription factor Xvent-2 in <i>Xenopus laevis</i> embryogenesis

“…The mRNA was found along the edge of the neural tube [3,5,6,9] along the edge of the presumptive forebrain [13], in the presumptive eyes, bronchial arch, tail bud [3][4][5][6] somites [3] and in the areas of the mieloid ventral blood island [4,5,25]. The Xvent-2 mRNA was absent in the cement gland [3][4][5][6].…”

“…The presence of the homeodomain and its specific sequence certificate this protein to be a transcription factor of a BarH subfamily. This gene is a target of the BMP signaling pathway in Xenopus and plays an important role in the dorsoventral patterning in amphibia and fish [3][4][5][6][7][8][9][10][11][12][13].…”