Wnt3a/β-catenin signaling controls posterior body development by coordinating mesoderm formation and segmentation

Abstract

Somitogenesis is thought to be controlled by a segmentation clock, which consists of molecular oscillators in the Wnt3a, Fgf8 and Notch pathways. Using conditional alleles of Ctnnb1 (␤-catenin), we show that the canonical Wnt3a/␤-catenin pathway is necessary for molecular oscillations in all three signaling pathways but does not function as an integral component of the oscillator. Small, irregular somites persist in abnormally posterior locations in the absence of ␤-catenin and cycling clock gene expression. Conversely, Notch pathway genes continue to oscillate in the presence of stabilized ␤-catenin but boundary formation is delayed and anteriorized. Together, these results suggest that the Wnt3a/␤-catenin pathway is permissive but not instructive for oscillating clock genes and that it controls the anterior-posterior positioning of boundary formation in the presomitic mesoderm (PSM). The Wnt3a/␤-catenin pathway does so by regulating the activation of the segment boundary determination genes Mesp2 and Ripply2 in the PSM through the activation of the Notch ligand Dll1 and the mesodermal transcription factors T and Tbx6. Spatial restriction of Ripply2 to the anterior PSM is ensured by the Wnt3a/␤-catenin-mediated repression of Ripply2 in posterior PSM. Thus, Wnt3a regulates somitogenesis by activating a network of interacting target genes that promote mesodermal fates, activate the segmentation clock, and position boundary determination genes in the anterior PSM.

FGF4 and FGF8 comprise the wavefront activity that controls somitogenesis

“…3 G-J), suggesting that the WNT pathway is downstream of FGF signaling. Conversely, loss of WNT signaling, which similarly halts axis extension and somitogenesis, reciprocally results in the loss of FGF gene expression (13,16). This reciprocal and complementary down-regulation prevents the determination of which signal is the wavefront.…”

“…The recombined allele produces constitutively active β-catenin in the PSM, producing "WNT GOF" embryos (13,16). By introducing this allele into the FGF mutant background (T-Cre; Fgf4 flox/null ; Fgf8 flox/null ; Ctnnb1 lox(ex3)/+ ), we maintained canonical WNT signaling in the PSM of FGF mutants, generating "WNTrestored" embryos.…”

“…Manipulation of canonical WNT signaling also causes corresponding shifts in the determination front (13,15,16). However, in WNT loss-of-function embryos, FGF signaling is also affected.…”

The Evolution of the Wnt Pathway

“…Similarly to the posterior growth zone in amphioxus tail buds (Schubert et al 2001), nuclear b-catenin also forms a posterior-to-anterior gradient in vertebrates. Wnt signaling is involved in tail development in zebrafish (Agathon et al 2003;Shimizu et al 2005;Thorpe et al 2005), where Wnt3a determines the anterior-posterior position of the somite determination front (Aulehla et al 2003;Dunty et al 2008;De Robertis 2010;Niehrs 2010). …”

Wnt Signaling in Mammalian Development: Lessons from Mouse Genetics

“…Secondly, a gradient of Wnt signaling activity, likely established by the graded expression of Wnt3a along the A-P axis, is thought to constitute part of the "determination wavefront" that positions the segmentation boundary during somitogenesis (Pourquie 2011). By manipulating Wnt activity gradient using b-catenin LOF and GOF mutants, studies show that high level Wnt activity maintains posterior PSM in an undifferentiated state permissive of Notch signaling oscillation, while a gradual reduction of Wnt activity in the anterior PSM sets the boundary to arrest Notch signaling oscillation and to activate genes required for segment formation (Aulehla et al 2008;Dunty et al 2008). Wnt3a is also required for left-right (L-R) axis determination because Wnt3a null mutants display laterality defects in multiple visceral organs (Nakaya et al 2005).…”