Canonical and Non-Canonical Wnt Signaling Generates Molecular and Cellular Asymmetries to Establish Embryonic Axes

Abstract

The formation of embryonic axes is a critical step during animal development, which contributes to establishing the basic body plan in each particular organism. Wnt signaling pathways play pivotal roles in this fundamental process. Canonical Wnt signaling that is dependent on β-catenin regulates the patterning of dorsoventral, anteroposterior, and left-right axes. Non-canonical Wnt signaling that is independent of β-catenin modulates cytoskeletal organization to coordinate cell polarity changes and asymmetric cell movements. It is now well documented that components of these Wnt pathways biochemically and functionally interact to mediate cell-cell communications and instruct cellular polarization in breaking the embryonic symmetry. The dysfunction of Wnt signaling disrupts embryonic axis specification and proper tissue morphogenesis, and mutations of Wnt pathway genes are associated with birth defects in humans. This review discusses the regulatory roles of Wnt pathway components in embryonic axis formation by focusing on vertebrate models. It highlights current progress in decoding conserved mechanisms underlying the establishment of asymmetry along the three primary body axes. By providing an in-depth analysis of canonical and non-canonical pathways in regulating cell fates and cellular behaviors, this work offers insights into the intricate processes that contribute to setting up the basic body plan in vertebrate embryos.

Keywords: Spemann organizer; Wnt signaling; anteroposterior axis; asymmetry formation; axis formation; dorsoventral axis; laterality defects; left–right axis; left–right organizer.

Figure 1

Wnt signaling pathways in vertebrates. (A) In the Wnt/β-catenin pathway, the binding of ligands to Fzd receptors and LRP5/6 co-receptors leads to the stabilization of β-catenin and transcription of target genes. The E3 ubiquitin ligases RNF43 and ZNRF3 function to regulate the lysosomal degradation of Fzd receptors by promoting their ubiquitination (Ub). This activity is antagonized by the binding of R-spondins to LGR4/5/6. (B) Wnt/PCP signaling is induced and propagated through the interaction between non-canonical Wnts and receptor–co-receptor complexes (Fzd/Ror1/2 or Fzd/Ryk) as well as by the asymmetric localization of “core” PCP proteins. The signal is relayed by downstream effectors which regulate cytoskeletal rearrangements or activate transcriptional responses. (C) The Wnt/Ca²⁺ branch activates PLC through heteromeric G proteins to trigger calcium-dependent cytoskeletal changes and NFAT-mediated target gene transcription. Dvl proteins contribute to activating different Wnt pathways through distinct domains: N-terminal DIX, central PDZ, C-terminal DEP, and extreme C-terminus. It should be noted that although Ryk and Ror are often associated with controlling polarized cell behaviors, they may be also involved in modulating canonical Wnt signaling [23].

Figure 2

Simplified model of Wnt/β-catenin signaling in the specification of D-V and A-P axes during Xenopus development. (A) At cleavage stages, maternal Hwa and β-catenin (β-cat) are accumulated in the dorsal–vegetal region as a result of cortical rotation and selective protection. The dorsal-vegetal blastomeres with high levels of β-catenin and Nodal proteins constitute the Nieuwkoop center. Activation of maternal Wnt/β-catenin signaling will induce the formation of the Spemann organizer after zygotic transcription. (B) In the gastrula, the Spemann organizer region secretes extracellular inhibitors for Wnts and BMPs to prevent their ventralizing activity. This antagonistic interaction patterns the D-V axis. (C) During and after gastrulation, Wnt/β-catenin signaling is involved in A-P patterning, with higher activity at the posterior region of the embryo.

Figure 3

Schematic of asymmetric cellular behaviors regulated by Wnt/PCP signaling in A-P axis elongation. (A) Mediolateral cell intercalation in CE movements during gastrulation narrows tissues along the mediolateral plane and elongates the embryo along the A-P axis. (B) Radial cell intercalation reduces the number of cell layers and drives tissue spreading.

Figure 4

Schematic of “core” PCP protein localization and L–R asymmetry formation in the mouse node. (A) The posterior expression of Wnt5a and Wnt5b in the node forms a gradient of Wnt/PCP signaling along the A-P axis to initiate the asymmetric localization of “core” PCP proteins. In the anterior region of the node, high levels of Wnt antagonists sFRP1/2/5 prevent Wnt/PCP signaling. The asymmetric localization of “core” PCP proteins contributes to restricting the posterior positioning of ciliary basal bodies (orange dots) in node cells. (B) At the dome-shaped apical surfaces of node cells, the posterior tilting and the clockwise rotational motion of motile cilia generate leftward fluid flow (blue arrows) within the node cavity, resulting in an increased calcium concentration on the left side (vertical yellow arrow). This Nodal flow triggers left-sided gene expression and breaks the bilateral symmetry.