Compartment-dependent activities of Wnt3a/β-catenin signaling during vertebrate axial extension

Abstract

Extension of the vertebrate body results from the concerted activity of many signals in the posterior embryonic end. Among them, Wnt3a has been shown to play relevant roles in the regulation of axial progenitor activity, mesoderm formation and somitogenesis. However, its impact on axial growth remains to be fully understood. Using a transgenic approach in the mouse, we found that the effect of Wnt3a signaling varies depending on the target tissue. High levels of Wnt3a in the epiblast prevented formation of neural tissues, but did not impair axial progenitors from producing different mesodermal lineages. These mesodermal tissues maintained a remarkable degree of organization, even within a severely malformed embryo. However, from the cells that failed to take a neural fate, only those that left the epithelial layer of the epiblast activated a mesodermal program. The remaining tissue accumulated as a folded epithelium that kept some epiblast-like characteristics. Together with previously published observations, our results suggest a dose-dependent role for Wnt3a in regulating the balance between renewal and selection of differentiation fates of axial progenitors in the epiblast. In the paraxial mesoderm, appropriate regulation of Wnt/β-catenin signaling was required not only for somitogenesis, but also for providing proper anterior-posterior polarity to the somites. Both processes seem to rely on mechanisms with different requirements for feedback modulation of Wnt/β-catenin signaling, once segmentation occurred in the presence of high levels of Wnt3a in the presomitic mesoderm, but not after permanent expression of a constitutively active form of β-catenin. Together, our findings suggest that Wnt3a/β-catenin signaling plays sequential roles during posterior extension, which are strongly dependent on the target tissue. This provides an additional example of how much the functional output of signaling systems depends on the competence of the responding cells.

Tet proteins influence the balance between neuroectodermal and mesodermal fate choice by inhibiting Wnt signaling

“…6 I, Left). Timing and duration of Wnt activity are important parameters for the induction and maintenance of NMPs, which maintain selfrenewal and bipotency at moderate levels of Wnt signaling: Cells exposed to a low-Wnt environment become neural, whereas cells exposed to high levels of Wnt transition to a mesodermal fate (47,50,51). Our studies of Tet1/2/3-deficient embryos and Tet3-deficient mESCs indicated that Wnt signaling was abnormally activated in the absence of TET proteins, particularly Tet3.…”

“…In Cdx2P-Wnt3a embryos, neuroectodermal specification is adversely affected, as evidenced by strong downregulation of Sox2. However, activation of Wnt3a does not result in overproduction of paraxial mesoderm, but, rather, resulted in ectopic lateral mesoderm expressing Wnt2 and Tbx4 (47). Previous studies have demonstrated that specification of lateral mesoderm at the expense of paraxial mesoderm occurs in the absence of signaling from the neural tube, offering an explanation as to why lateral mesodermal fate, rather than paraxial mesodermal fate, is observed with loss of neural specification (49).…”

“…Activation of Wnt signaling can prevent NMPs from adopting a neural cell fate, because high Wnt3a levels in PS down-regulate Sox2 expression (47). To examine whether Wnt activity was altered in Tet1/2/3 TKO embryos, we analyzed expression of the active, dephosphorylated form of β-catenin.…”

Independent regulation of vertebral number and vertebral identity by microRNA-196 paralogs

“…The repression of Wnt by trunk Hox genes is less intuitive because, at least on a Cdx2/4 mutant background, ectopic Hoxb8 has been shown to maintain Wnt expression in the tailbud (29). However, the importance of precise Wnt levels in the early steps of axis formation, and of cellular context, are beginning to be appreciated (59,70) and may underlie such discrepancies. Exactly how miR-196 activity integrates within the genetic networks controlling PM progenitor maintenance and differentiation remains to be fully elucidated.…”

“…Vertebral progenitors in the epiblast and tailbud are sensitive to the levels of Wnt signaling. Genetic removal of the Wnt3a ligand (58), or conversely, ectopic activation of Wnt3a in the epiblast (59), result in severe axis truncation posterior to the forelimb. Wnt3a expression has been shown to decrease as progenitor cells commit to a paraxial mesoderm fate (60,61), and sustained Wnt activity disrupts somite formation (51) and somite polarity (59), dependent on timing and method of activation.…”