Anteroposterior axis patterning by early canonical Wnt signaling during hemichordate development

Abstract

The Wnt family of secreted proteins has been proposed to play a conserved role in early specification of the bilaterian anteroposterior (A/P) axis. This hypothesis is based predominantly on data from vertebrate embryogenesis as well as planarian regeneration and homeostasis, indicating that canonical Wnt (cWnt) signaling endows cells with positional information along the A/P axis. Outside of these phyla, there is strong support for a conserved role of cWnt signaling in the repression of anterior fates, but little comparative support for a conserved role in promotion of posterior fates. We further test the hypothesis by investigating the role of cWnt signaling during early patterning along the A/P axis of the hemichordate Saccoglossus kowalevskii. We have cloned and investigated the expression of the complete Wnt ligand and Frizzled receptor complement of S. kowalevskii during early development along with many secreted Wnt modifiers. Eleven of the 13 Wnt ligands are ectodermally expressed in overlapping domains, predominantly in the posterior, and Wnt antagonists are localized predominantly to the anterior ectoderm in a pattern reminiscent of their distribution in vertebrate embryos. Overexpression and knockdown experiments, in combination with embryological manipulations, establish the importance of cWnt signaling for repression of anterior fates and activation of mid-axial ectodermal fates during the early development of S. kowalevskii. However, surprisingly, terminal posterior fates, defined by posterior Hox genes, are unresponsive to manipulation of cWnt levels during the early establishment of the A/P axis at late blastula and early gastrula. We establish experimental support for a conserved role of Wnt signaling in the early specification of the A/P axis during deuterostome body plan diversification, and further build support for an ancestral role of this pathway in early evolution of the bilaterian A/P axis. We find strong support for a role of PLOS Biology | https://doi.org/10.1371/journal.pbio

New roles for Wnt and BMP signaling in neural anteroposterior patterning

“…Recent findings in the hemichordate, Saccoglossus kowalevskii (Acorn worm) support our observations [75]. In the acorn worm, canonical Wnt signaling represses anterior and induces mid-axial ectodermal fates during early development.…”

Untangling posterior growth and segmentation by analyzing mechanisms of axis elongation in hemichordates

“…The Role of cWnt Signaling During PAE in S. kowalevskii. In S. kowalevskii multiple Wnt ligands are expressed posteriorly in broad territories and multiple germ layers (24). To test whether cWnt signaling plays a role in regulating PAE in S. kowalevskii, we injected RNA of the Wnt-signaling inhibitor, secreted frizzled receptor protein (sfrp1/5), to broadly down-regulate frizzledmediated Wnt signaling in the embryo (24).…”

Apcdd1 is a dual BMP/Wnt inhibitor in the developing nervous system and skin

“…Apcdd1 is similar to orthologous genes in several other genomes, but to no other genes, suggesting Apcdd1 is a taxon-restricted gene 20 that may have arisen at the origin of Metazoa. Interestingly, Apcdd1 was maintained in a few metazoan phyla where it acquired useful functions in Wnt and BMP signaling, which in these phyla control axial specification during embryonic development (Angerer et al, 2000;Darras et al, 2018;Guder et al, 2006;Henry et al, 2010;Henry et al, 2008;Hogvall et al, 2014;Kuo and Weisblat, 2011;Lambert et al, 2016;Lowe et al, 2006;Marlow et al, 2014;Molina et al, 2011;25 Pang et al, 2011;Pang et al, 2010;Park and Priess, 2003;Plickert et al, 2006;Rentzsch et al, 2007;Schenkelaars et al, 2017;Scimone et al, 2016;Suzuki et al, 1999;Treffkorn and Mayer, 2013;Wikramanayake et al, 1998;Windsor and Leys, 2010) and was lost in phyla that do not utilize Bmp for axial specification have lost APCDD1 (Fig. 5A, Supp.…”

“…Apcdd1 protein sequence and secondary structure analysis. To analyze the content of secondary structure elements within Apcdd1 proteins, we used bioinformatics algorithm collections PredictProtein (www.predictprotein.org) and HMMER (hmmer.org) (Eddy, 1998;Yachdav et al, 2014). Within 441 Apcdd1 proteins from different Metazoan species, we scored sequence features and predicted secondary structural properties according to over 25 criteria.…”