A conserved function for Strabismus in establishing planar cell polarity in the ciliated ectoderm during cnidarian larval development

Abstract

SUMMARYFunctional and morphological planar cell polarity (PCP) oriented along the oral-aboral body axis is clearly evident in the ectoderm of torpedo-shaped planula larvae of hydrozoan cnidarians such as Clytia hemisphaerica. Ectodermal epithelial cells bear a single motile cilium the beating of which is coordinated between cells, causing directional swimming towards the blunt, aboral pole. We have characterised PCP during Clytia larval development and addressed its molecular basis. PCP is first detectable in ectodermal cells during gastrulation as coordinated basal body positioning, the ciliary root becoming consistently positioned on the oral side of the apical surface of the cell. At later stages, more pronounced structural polarity develops around the base of each cilium in relation to the cilia beating direction, including a characteristic asymmetric cortical actin organisation. Morpholino antisense oligonucleotide and mRNA injection studies showed that PCP development requires the Clytia orthologues of the core Fz-PCP pathway components Strabismus (CheStbm), Frizzled (CheFz1) and Dishevelled (CheDsh). Morpholinos targeting any of these components prevented ectodermal PCP, disrupted ciliogenesis and inhibited embryo elongation during gastrulation, which involves cell intercalation. We show that YFP-tagged CheStbm adopts a polarised intracellular distribution, localising preferentially to the aboral boundary of each cell, as has been demonstrated in Drosophila and some vertebrate PCP studies. Our findings in a cnidarian strongly suggest that the Fz-PCP pathway is a highly conserved and evolutionary ancient metazoan feature that is probably widely responsible for oriented swimming and/or feeding in relation to body axis in the many ciliated larval types found throughout the animal kingdom.

Differential Responses to Wnt and PCP Disruption Predict Expression and Developmental Function of Conserved and Novel Genes in a Cnidarian

“…Fz1 acts as a receptor for Wnt3 to activate Wnt/β-catenin signaling [39], [40], but is also thought to interact with the Clytia Strabismus protein to mediate planar cell polarity (PCP), necessary for cell alignment in the ectoderm but also axial elongation during larval development and endoderm formation [41]. We thus hypothesized that the differences in expression responses in Fz1-MO versus Wnt3-MO could be due to the specific involvement of Fz1 in PCP.…”

“…This activation establishes distinct regional identities characterized by specific sets of transcribed genes at the oral and aboral poles of the developing embryo, including those required for cell ingression at gastrulation. Fz-PCP signaling, dependent on the conserved transmembrane protein Strabismus (Stbm), is activated in parallel along the same axis to coordinate cell polarity in the ectoderm and to guide embryo elongation [41]. Since multi-member Wnt families with early polarized embryonic expression have also been uncovered in other cnidarians [42], [43], ctenophores and sponges [44]–[47] as well as in a range of bilaterian models [48], [49], it seems highly probable that Wnt/Fz signaling regulated embryonic patterning in ancestral metazoans, specifying the primary body axes and/or presumptive germ layer regions.…”

Bbof1 is required to maintain cilia orientation

“…A similar planar localization of PCP components occurs in epithelial cells with motile cilia (Guirao et al, 2010;Momose et al, 2012;Vladar et al, 2012), and this pattern likely arises prior to cilogenesis . Altering PCP signaling causes cilia misorientation, as seen by a misalignment of cells along a tissue axis, rather than by a misalignment of cilia within a given cell (Guirao et al, 2010;Mitchell et al, 2009;Momose et al, 2012;Vladar et al, 2012). Moreover, as perturbing PCP components can influence the planar orientation of ciliated cells non-cellautonomously, it is likely that PCP acts at sites of cell-cell contact to cue planar polarity (Mitchell et al, 2009).…”

“…Proteins in the PCP pathway localize differentially in epithelial cells at sites of cell-cell contact, adopting a head-to-tail arrangement along the planar axis (Bayly and Axelrod, 2011;Zallen, 2007). A similar planar localization of PCP components occurs in epithelial cells with motile cilia (Guirao et al, 2010;Momose et al, 2012;Vladar et al, 2012), and this pattern likely arises prior to cilogenesis . Altering PCP signaling causes cilia misorientation, as seen by a misalignment of cells along a tissue axis, rather than by a misalignment of cilia within a given cell (Guirao et al, 2010;Mitchell et al, 2009;Momose et al, 2012;Vladar et al, 2012).…”

Cnidaria

“…In strabismus morphants, apart from the disorganization of the cell polarity in the cell sheet refl ected in the chaotic orientation of cilia, the embryos also failed to elongate during gastrulation, and the gastrulation site was expanded (Momose et al 2012 ). A similar lack of elongation in the CheFz1 , CheWnt3 , and ChDsh ( dishevelled ) morphants suggests a link between the canonical Wnt signaling via CheWnt3, which might serve as a cue for the initial orientation of planar cell polarity (Momose et al 2012 ). Clytia also provided new insight into striated muscle evolution, with implications for the hypothesis that the entocodon is homologous to mesoderm in Bilateria.…”