Silencing ofSmed-βcatenin1generates radial-like hypercephalized planarians

Abstract

Little is known about the molecular mechanisms responsible for axis establishment during non-embryonic processes such as regeneration and homeostasis. To address this issue, we set out to analyze the role of the canonical Wnt pathway in planarians, flatworms renowned for their extraordinary morphological plasticity. Canonical Wnt signalling is an evolutionarily conserved mechanism to confer polarity during embryonic development, specifying the anteroposterior (AP) axis in most bilaterians and the dorsoventral (DV) axis in early vertebrate embryos. β-Catenin is a key element in this pathway, although it is a bifunctional protein that is also involved in cell-cell adhesion. Here, we report the characterization of two β-catenin homologs from Schmidtea mediterranea(Smed-βcatenin1/2). Loss of function of Smed-βcatenin1, but not Smed-βcatenin2, in both regenerating and intact planarians, generates radial-like hypercephalized planarians in which the AP axis disappears but the DV axis remains unaffected, representing a unique example of a striking body symmetry transformation. The radial-like hypercephalized phenotype demonstrates the requirement for Smed-βcatenin1 in AP axis re-establishment and maintenance, and supports a conserved role for canonical Wnt signalling in AP axis specification, whereas the role of β-catenin in DV axis establishment would be a vertebrate innovation. When considered alongside the protein domains present in each S. mediterranea β-catenin and the results of functional assays in Xenopus embryos demonstrating nuclear accumulation and axis induction with Smed-βcatenin1, but not Smed-βcatenin2, these data suggest that S. mediterraneaβ-catenins could be functionally specialized and that only Smed-βcatenin1 is involved in Wnt signalling.

Localization of planarian β-CATENIN-1 reveals multiple roles during anterior-posterior regeneration and organogenesis

“…notum and sFRP) localize to the anterior pole, and their inhibition leads to anteriorized or posteriorized phenotypes, respectively (Adell et al, 2009;Gurley et al, 2010;Lander and Petersen, 2016;Reddien, 2009b, 2011;Scimone et al, 2016;Sureda-Gómez et al, 2015). Interestingly, recent reports have suggested that β-catenin-1 exerts additional roles in eye and brain patterning (Hill and Petersen, 2015;Owen et al, 2015), which is consistent with the ubiquitous mRNA expression of β-catenin-1 (Gurley et al, 2008;Iglesias et al, 2008;Petersen and Reddien, 2008). To date, however, no studies have described the localization of β-CATENIN-1 protein either as an AP gradient or in specific developing tissues.…”

“…This model, which departs from classic regeneration experiments (Child, 1911(Child, , 1941Morgan, 1904Morgan, , 1905, is further supported by the range of phenotypes generated after β-catenin-1 inhibition, in which anterior and posterior structures are gradually affected according to the dose and time of inhibition Iglesias et al, 2008). These phenotypes vary from loss of the most posterior structures ('tail-less' phenotype) to 'radial-like' hypercephalization with no posterior or central identity Iglesias et al, 2008). Moreover, it is known that several Wnt ligands are expressed posteriorly, whereas inhibitors (e.g.…”

“…Their striking regenerative capacity coupled with the continuous remodeling of their tissues to adapt their body size to food availability (Saló, 2006) allows analysis of signaling pathways in the intact organism in multiple situations. The planarians Schmidtea mediterranea and Schmidtea polychroa have two β-catenin paralogs, namely β-catenin-1 and β-catenin-2 (Gurley et al, 2008;Iglesias et al, 2008;Martín-Durán and Romero, 2011;Martín-Durán et al, 2010). While β-catenin-2 is involved in cell-cell adhesions (Chai et al, 2010), inhibition of planarian β-catenin-1 results in a striking 'radial-like' hypercephalyzed phenotype, revealing a central role for canonical Wnt signaling in the specification of posterior identities during adult regeneration and homeostasis (Gurley et al, 2008;Iglesias et al, 2008;Petersen and Reddien, 2008).…”

“…The planarians Schmidtea mediterranea and Schmidtea polychroa have two β-catenin paralogs, namely β-catenin-1 and β-catenin-2 (Gurley et al, 2008;Iglesias et al, 2008;Martín-Durán and Romero, 2011;Martín-Durán et al, 2010). While β-catenin-2 is involved in cell-cell adhesions (Chai et al, 2010), inhibition of planarian β-catenin-1 results in a striking 'radial-like' hypercephalyzed phenotype, revealing a central role for canonical Wnt signaling in the specification of posterior identities during adult regeneration and homeostasis (Gurley et al, 2008;Iglesias et al, 2008;Petersen and Reddien, 2008). In contrast, inhibition of negative regulators of the canonical Wnt pathway, such as the components of the β-catenin destruction complex, APC and axin, leads to the duplication of posterior territories (i.e.…”

Bipolarity in planarians is not induced by space travel

“…It has been suggested that β ‐ catenin plays a key role in polarity specification in planarians (Gurley, Rink, & Alvarado, 2008; see also Iglesias, Gomez‐Skarmeta, Saló, & Bartscherer, 2008). We doubt that this presumed molecular switch (Gurley et al., 2008) is restricted to blastemas arising at only a few regions along the anterior−posterior axis of the planarian body (see also Morgan, 1904, who reported that heteromorphic heads may appear at any portion of the worm).…”