A Wnt5 Activity Asymmetry and Intercellular Signaling via PCP Proteins Polarize Node Cells for Left-Right Symmetry Breaking

Abstract

Polarization of node cells along the anterior-posterior axis of mouse embryos is responsible for left-right symmetry breaking. How node cells become polarized has remained unknown, however. Wnt5a and Wnt5b are expressed posteriorly relative to the node, whereas genes for Sfrp inhibitors of Wnt signaling are expressed anteriorly. Here we show that polarization of node cells is impaired in Wnt5aWnt5b and Sfrp mutant embryos, and also in the presence of a uniform distribution of Wnt5a or Sfrp1, suggesting that Wnt5 and Sfrp proteins act as instructive signals in this process. The absence of planar cell polarity (PCP) core proteins Prickle1 and Prickle2 in individual cells or local forced expression of Wnt5a perturbed polarization of neighboring wild-type cells. Our results suggest that opposing gradients of Wnt5a and Wnt5b and of their Sfrp inhibitors, together with intercellular signaling via PCP proteins, polarize node cells along the anterior-posterior axis for breaking of left-right symmetry.

Left–Right Patterning: Breaking Symmetry to Asymmetric Morphogenesis

“…A range of vertebrates also exhibit motile cilia within LROs where they generate L-R asymmetric fluid flows that drive asymmetries in signaling molecule activities around the periphery of LROs [58, 61–63]. In mouse and Xenopus , LRO cilia are polarized such that they are positioned to the posterior side of LRO cells, a process which depends on core planar cell polarity components [64] and, at least in mouse embryos, a gradient of WNT5 activity across the node in the A-P dimension [65]. This positioning imparts a posterior tilt to the cilia which, owing to the clockwise rotation of cilia when viewed ventrally, results in an ineffective rightward stroke near the cell surface and an effective leftward stroke that drives fluid flow across the node towards the left side (Figure I).…”

Wnt/PCP Instructions for Cilia in Left-Right Asymmetry

“…These data suggest that the Wnt activity gradients instruct basal body/cilia positioning via polarization of the core Fz/PCP proteins. Importantly, Minegishi and colleagues (2017) also demonstrate that Wnt5a is sufficient to redirect PCP orientation and basal body/cilia positioning when mis-expressed. They find that ectopic single-cell expression of Wnt5a redirects core PCP asymmetry and basal body/cilia positioning toward the Wnt5-expressing cell and that uniform Wnt5a expression in the node causes randomization of basal body/cilia positioning.…”

“…Reporting in Developmental Cell , Minegishi et al (2017) uncover an instructive link from Wnt5a/b gradients to PCP-factor-regulated polarized cilia positioning that is essential to mouse embryo left-right asymmetry establishment. …”

“…In this issue of Developmental Cell , Minegishi and colleagues (2017) address whether Wnt signaling gradients affect the positioning of basal bodies and assoiated cilia to the posterior region of node cells in the mouse. The precise posterior positioning of cilia in the node is critical for left-right (L/R) asymmetry establishment through coordinated beating (clockwise rotational motion) of properly aligned cilia and the associated directional flow of L/R determinants (Nonaka et al, 2005).…”

“…From the work of Minegishi et al (2017), a second conserved link emerges with regard to centriole positioning. Basal bodies are specialized centrioles, and their positioning dependence on Wnt/PCP echoes the recent demonstration that centriole positioning is regulated in response to asymmetric localization of core Fz/PCP complexes in Drosophila wings (Carvajal-Gonzalez et al, 2016a), where centrioles are located to the side of cells that is enriched in the Fz-Dsh complex and away from the Vang-Pk complex (Figure 1B).…”