Visceral Endoderm Mediates Forebrain Development by Suppressing Posteriorizing Signals

Abstract

The anterior visceral endoderm (AVE) has attracted recent attention as a critical player in mouse forebrain development and has been proposed to act as "head organizer" in mammals. However, the precise role of the AVE in induction and patterning of the anterior neuroectoderm is not yet known. Here we identified a 5'-flanking region of the mouse Otx2 gene (VEcis) that governs the transgene expression in the visceral endoderm. In transgenic embryos, VEcis-active cells were found in the distal visceral endoderm at 5.5 days postcoitus (dpc), had begun to move anteriorly at 5.75 dpc, and then became restricted to the AVE prior to gastrulation. The VEcis-active visceral endoderm cells exhibited ectodermal morphology distinct from that of the other endoderm cells and consisted of two cell layers at 5.75 dpc. In the Otx2(-/-) background, the VEcis-active endoderm cells remained distal even at 6.5 dpc when a primitive streak was formed; anterior definitive endoderm was not formed nor were any markers of anterior neuroectoderm ever induced. The Otx2 cDNA transgene under the control of the VEcis restored these Otx2(-/-) defects, demonstrating that Otx2 is essential to the anterior movement of distal visceral endoderm cells. In germ-layer explant assays between ectoderm and visceral endoderm, the AVE did not induce anterior neuroectoderm markers, but instead suppressed posterior markers in the ectoderm; Otx2(-/-) visceral endoderm lacked this activity. Thus Otx2 is also essential for the AVE to repress the posterior character. These results suggest that distal visceral endoderm cells move to the future anterior side to generate a prospective forebrain territory indirectly, by preventing posteriorizing signals.

The Wnt co-receptors Lrp5 and Lrp6 are essential for gastrulation in mice

“…Hesx1 expression is reportedly absent in Wnt3 mutants, and this observation has led to the hypothesis that the AVE is not sufficient to induce anterior neurectoderm (Liu et al, 1999). Based on transplant and explant studies, AVE signals are instead believed to repress posteriorizing signals from the primitive streak, thereby priming the epiblast to respond to additional anterior signals emanating from the anterior primitive streak (Tam and Steiner, 1999;Kimura et al, 2000) (reviewed by Lu et al, 2001;Perea-Gomez et al, 2001). The expression of Hesx1 in the epiblast of Lrp5/6 double mutants and mesd mutants suggests that the AVE is, in fact, sufficient to induce anterior neurectodermal cell fates in the absence of a primitive streak.…”

Crucial roles ofFoxa2in mouse anterior–posterior axis polarization via regulation of anterior visceral endoderm-specific genes

“…Expression in AVE. A 5Ј 1.8-kb mouse Otx2 promoter region directs expression in the DVE at E5.5 and in the AVE from the pre-to the midstreak stages (11). To identify the specific cis elements necessary for AVE expression in this region, a series of deletion constructs was generated and tested for activity in transgenic mice (Fig.…”

The DVE changes distal epiblast fate from definitive endoderm to neurectoderm by antagonizing nodal signaling

“…2G'). Otx2 has an essential function in restricting expression of Nodal (Perea-Gomez et al, 2001) and Cripto (Kimura et al, 2000) to the primitive streak region. Thus, this aspect of the phenotype of DVE-ablated embryos is entirely consistent with the existing model on the role of the DVE in general and Otx2 in particular.…”

“…In Cripto (Ding et al, 1998) and Otx2 (Acampora et al, 1998;Rhinn et al, 1998) mutants, the primitive streak is ectopically formed at the proximal portion of the embryo. These observations suggest that the DVE/AVE decides the position of the primitive streak (Kimura et al, 2000). However, because these mutant embryos maintain the AVE itself, additional role(s) of this tissue might not have been revealed, if the AVE was still functional in part.…”