At early stages of mammalian development, an embryo undergoes biochemical and anatomical changes that determine axis polarity. This architectural asymmetry is critical for the ultimate development of the full body plan. In the mouse, one such change occurs prior to gastrulation and includes a transformation of the embryonic axis from a proximal/distal (P/D) orientation to an anterior/posterior (A/P) orientation. Axis conversion is accompanied by the migration of distal visceral endoderm (DVE) cells located at the tip of the E5.5 egg cylinder, to the presumptive anterior region of the E6.0 embryo (Figure 1A).1,2 Here the DVE is termed anterior visceral endoderm (AVE) and is involved in the development of anterior structures.1
The molecular mechanisms underlying these cellular movements continue to be elucidated. For instance, inhibitors of Nodal signaling, Lefty and Cerberus-like (Cerl), appear to direct migration by a mechanism that involves regulating differential proliferation of the visceral endoderm along the future A/P axis.3,4 In addition, the transcription factor Otx-2 plays a pivotal role. In Otx-2 knockout mice the DVE forms, but cells do not migrate and the A/P axis fails to develop.5 A recent study by Kimura-Yoshida et al. provides new clues regarding Otx-2-directed DVE cell migration and describes downstream players involved in this important aspect of early mouse development.6
|Figure 1 A
|Figure 1 B
|Figure 1 A:
Specialized DVE cells are located at the tip of the wild-type E5.5 mouse embryo. As the embryo matures, these cells migrate to the presumptive anterior region. Here they are termed AVE, and are involved in the development of the A/P axis. Molecules directing these cellular movements include Wnt, localized to the presumptive posterior region of the E6.0 embryo, and its inhibitor Dkk, expressed in the AVE.
Mislocalization of Dkk-1 or Wnt-3a, using ectopic placement of protein-bound agarose
beads, caused DVE cells to incorrectly migrate to the future posterior side of the embryo.
Dickkopf 1 (Dkk-1), a classical inhibitor of canonical Wnt signaling pathways, is normally expressed in the presumptive anterior region of the visceral endoderm at the time of DVE migration (E5.75). However, it is not expressed in Otx-2-/- mice.6-8 This suggests that Otx-2 regulation of Dkk-1, and subsequently of Wnt signaling, might underlie A/P axis formation.6 Using genetic and biochemical approaches, the authors provide evidence that this is indeed the case. In Otx-2-/- mice, targeted knockin of Dkk-1 restores normal DVE migration patterns and AP axis formation. To determine whether this was due to Dkk-1-mediated Wnt inhibition, Wnt-8a, normally expressed in the presumptive posterior region, was mislocalized throughout the E6.0 epiblast. This manipulation resulted in the subsequent misexpression of β-Catenin (a Wnt effector) throughout the visceral endoderm, and prevented DVE migration and the development of A/P polarity.
These results suggest that Wnt activity dominates on the future posterior side and Dkk-1 is active anteriorly, resulting in opposing biochemical signals that guide DVE movement. How might asymmetrical Wnt signaling regulate these processes? The Wnt family and associated canonical signaling cascades have the ability to affect development via multiple mechanisms including regulating cell fate, migration, and cell growth/survival. The authors present evidence that Wnt and Dkk are acting as guidance cues, balancing attractive and repulsive signals in order to guide DVE cells to their final anterior position. Targeted application of Wnt-3a or Dkk-1 with protein-bound agarose beads showed that DVE migration patterns could be manipulated, and even reversed, depending on whether the bead was placed in presumptive anterior or posterior regions of the embryo. Specifically, Wnt-3a or Dkk-1 mislocalization to the future anterior or posterior region, respectively, could cause misdirected DVE migration to the posterior side (Figure 1B).
These new studies provide evidence that regulation of canonical Wnt pathways plays a pivotal role in determining early mouse axis polarity. More specifically, asymmetrical molecular signals carried by Wnt and its inhibitor Dkk act as guidance cues, directing the migration of DVE cells and regulating the earliest stages of A/P axis formation.